chore: remove ReachabilitySwift from podspec and SPM

[lawmicha]

Issue #, if available:

Description of changes:

Reachability was removed in #521

> pod spec lint

 -> AWSAppSync (3.6.0)
    - NOTE  | url: The URL (http://aws.amazon.com/mobile/sdk) is not reachable.
    - NOTE  | xcodebuild:  note: Using new build system
    - NOTE  | xcodebuild:  note: Using codesigning identity override: -
    - NOTE  | xcodebuild:  note: Build preparation complete
    - NOTE  | [iOS] xcodebuild:  note: Planning
    - NOTE  | [iOS] xcodebuild:  note: Building targets in dependency order
    - NOTE  | [iOS] xcodebuild:  Pods.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 15.2.99. (in target 'Starscream' from project 'Pods')
    - NOTE  | [iOS] xcodebuild:  Pods.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 15.2.99. (in target 'SQLite.swift' from project 'Pods')
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/ApolloClient.swift:5:30: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/ApolloStore.swift:7:33: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Internal/AppSyncSubscriptionWithSync.swift:522:42: warning: 'hash' is deprecated: Use hashData instead.
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/AWSAppSyncHTTPNetworkTransport.swift:209:46: warning: 'hash' is deprecated: Use hashData instead.
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/GraphQLOperation.swift:1:35: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/GraphQLResultAccumulator.swift:1:43: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/NetworkReachability.swift:17:47: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead

Analyzed 1 podspec.

[!] The spec did not pass validation, due to 7 warnings (but you can use `--allow-warnings` to ignore them).
> pod spec lint --allow-warnings

 -> AWSAppSync (3.6.0)
    - NOTE  | url: The URL (http://aws.amazon.com/mobile/sdk) is not reachable.
    - NOTE  | xcodebuild:  note: Using new build system
    - NOTE  | xcodebuild:  note: Using codesigning identity override: -
    - NOTE  | xcodebuild:  note: Build preparation complete
    - NOTE  | [iOS] xcodebuild:  note: Planning
    - NOTE  | [iOS] xcodebuild:  note: Building targets in dependency order
    - NOTE  | [iOS] xcodebuild:  Pods.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 15.2.99. (in target 'Starscream' from project 'Pods')
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/ApolloClient.swift:5:30: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/ApolloStore.swift:7:33: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Internal/AppSyncSubscriptionWithSync.swift:522:42: warning: 'hash' is deprecated: Use hashData instead.
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/AWSAppSyncHTTPNetworkTransport.swift:209:46: warning: 'hash' is deprecated: Use hashData instead.
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/GraphQLOperation.swift:1:35: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/Apollo/Sources/Apollo/GraphQLResultAccumulator.swift:1:43: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - WARN  | xcodebuild:  AWSAppSync/AWSAppSyncClient/NetworkReachability.swift:17:47: warning: using 'class' keyword to define a class-constrained protocol is deprecated; use 'AnyObject' instead
    - NOTE  | [iOS] xcodebuild:  Pods.xcodeproj: warning: The iOS Simulator deployment target 'IPHONEOS_DEPLOYMENT_TARGET' is set to 8.0, but the range of supported deployment target versions is 9.0 to 15.2.99. (in target 'SQLite.swift' from project 'Pods')

Analyzed 1 podspec.

AWSAppSync.podspec passed validation.

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