chore: removing unused Gradle files in api-common-java, gax-java, java-common-protos, and java-iam (#1305)

The build is Maven or Bazel. The Gradle files in the recently migrated repositories (api-common-java, gax-java, java-common-protos, and java-iam) are not used.

Note that this pull request is not touching rules_java_gapic/resources/gradle which is still used to generate Gradle files for self-service client libraries.

Author: suztomo