[core] Delete gcs heartbeat manager

[fishbone]

Why are these changes needed?

The pull-based health check has been running for a month and no significant issue. This PR delete the old code to keep the codebase clean.

We'll maintain the new ones in the future.

Related issue number

Checks

• [ ] I've signed off every commit(by using the -s flag, i.e., git commit -s) in this PR.
• [ ] I've run scripts/format.sh to lint the changes in this PR.
• [ ] I've included any doc changes needed for https://docs.ray.io/en/master/.
• [ ] I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
  • [ ] Unit tests
  • [ ] Release tests
  • [ ] This PR is not tested :(

[ericl]

LGTM minus test failures

[fishbone]

let me fix the test.

[fishbone]

@ericl another look?