fix: remove unnecessary protocol conformance

[kneekey23]

Description of changes

This PR does two things to continue to shave off bytes:

1. Remove unnecessary Encodable & Reflection conformance by input types where there is no serializable body
2. Remove unnecessary generation of decodable structs where there is nothing in the response to deserialize

This shaves off .2 mb from Lambda as an example and decreases the number of files being generated.

Corresponding PR: awslabs/aws-sdk-swift#540

Scope

• [ x] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

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