Revert "Fix Naming/MemoizedInstanceVariableName offenses"

This reverts commit e36d683.

 This is because we've now disabled all the rules that would have been
`DisabledByDefault`. This is so that downstream consumers don't get any
nasty surprises. As a result, there should be no RuboCop linting
violations in the code in this repo, so we can back out the changes to
slim the diff: they're now redundant.

Of course, it would be a good idea to go and make these changes at a later
date, but we can do that when we enable the correct rules having matched
each one up with its rationale in our styleguide.

lib/rubocop/cop/github/insecure_hash_algorithm.rb

@@ -93,7 +93,7 @@ def just_encoding?(val)
         ].freeze
 
         def allowed_hash_functions
-          @allowed_hash_functions ||= cop_config.fetch("Allowed", DEFAULT_ALLOWED).map(&:downcase)
+          @allowed_algorithms ||= cop_config.fetch("Allowed", DEFAULT_ALLOWED).map(&:downcase)
         end
 
         def alg_name(val)