See https://github.com/grpc/grpc/issues/38210#issuecomment-2783948947 for context
Now that Event Engine is enabled, we no longer need the channel polling thread, so we can remove it.
Fork support (with event engine left enabled), is still broken on this PR as it is on master, but it will get cleaned up after https://github.com/grpc/grpc/pull/38980 (tested by manually merging and running fork tests)
While we're here:
- Regenerate greeter example protos
- Remove `channel_state_test.rb` in favor of `connectivity_watch_interrupted_test.rb`. The former was skipped due to https://bugs.ruby-lang.org/issues/15499, the new test changes things so that we no longer hit that bug, but still exercise the no_gvl connectivity state watch machinery.
- tweaks to fork tests (they are not running continuously but we should be able to re-enable them again after https://github.com/grpc/grpc/pull/38980 lands
Closes#39409
COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/39409 from apolcyn:fix_bugs 3e3ae7d81fcbd13b1d0eed6b13c7c39fe1bd1030
PiperOrigin-RevId: 766790187
