[kotlin][client] do not generate Serializable(forClass) annotation as the serializer is already defined

[hrach]

This removes excessive annotation that newly produces a warning that may prevent compilation of the generated code.

@Serializable(forClass=x) is intended for the Kotlin Serialization plugin to generate the serializable logic automatically. But since this generator generates this logic, there is nothing to be generated from the serialization plugin's side.

The error:

@Serializer annotation has no effect on class 'XXX', because all members of KSerializer are already overridden

Documentation: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/serializers.md#deriving-external-serializer-for-another-kotlin-class-experimental

@e5l

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
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• File the PR against the correct branch: master (upcoming 7.x.0 minor release - breaking changes with fallbacks), 8.0.x (breaking changes without fallbacks)
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[e5l]

thanks! Looks good as well. I would merge both PRs into one :)