fix(IoT): Fixing a intermittent crash when deallocating AWSIoTStreamThread

[sebaland]

Issue #, if available:

• AWSIoTStreamThread .cxx_destruct crashed #5258

Description of changes:
We're currently calling [self cleanUp] on dealloc, which basically invalidates the timer and checks if it should close the streams and invoke a callback.

Generally, it's not recommended to rely on the dealloc method to perform expensive operations. cleanUp is already always being called when the thread is cancelled, so calling it again when the thread is deallocated is not only redundant, but might be the cause of the linked crash due to attempting to access other near-deallocated objects and/or properties.

Check points:

• Added new tests to cover change, if needed
• All unit tests pass
• All integration tests pass
• Updated CHANGELOG.md
• Documentation update for the change if required
• PR title conforms to conventional commit style

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.