Stop using private CommonCrypto APIs on iOS

[graebm]

Issue: App Store submissions are being rejected due to use of non-public CommonCrypto functions for doing AES GCM:

• https://github.com/awslabs/aws-crt-swift/issues/206
• https://github.com/aws-amplify/amplify-swift/issues/3324

There are no public APIs for doing AES GCM via CommonCrypto

Description of changes: Remove the code for doing AES GCM (via non-public CommonCrypto functions) on iOS, watchOS, etc

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