chore(ci): remove legacy generate test

[noahdietz]

Removes the legacylibrarian Python container test for generate, which is no longer maintained or used.

The image build step is retained for build verification triggered by changes to the .generator/ directory. Unit tests otherwise cover the code therein.

[gemini-code-assist]

Code Review

This pull request simplifies the CI process by removing the library generation and building steps from cloudbuild-test.yaml, leaving only the generator image build verification. Additionally, two JSON metadata files for google-cloud-language were deleted. The reviewer suggests optimizing the build resources by reducing the timeout and machine type, and using the --no-push flag for Kaniko to avoid potential tag collisions during pull request builds.