Kotlin: clean up some unused symbols (#52029)

Summary:
Static code analysis reports numerous unused symbols across the codebase, which accounts for several static code analysis warnings. I've cleaned up some of them (not all of them because many of them are false positives from testing files)

The cleaned-up symbols are mostly private and from internal classes, so this should not impact users (unless they are used internally at Meta – in that case, let me know so I can revert accordingly)

## Changelog:

[INTERNAL] - Kotlin: clean up some unused symbols

Pull Request resolved: https://github.com/facebook/react-native/pull/52029

Test Plan:
```sh
yarn test-android
yarn android
```

Reviewed By: javache

Differential Revision: D76722313

Pulled By: cortinico

fbshipit-source-id: 8c7dfe204fa7c457b7484a7edd120ae45e1d604d