[core] Remove unused runtime env callback

[dentiny]

I don't see the callback within Deleter used anywhere.

[rynewang]

cpp tests build failure

[2024-12-29T12:19:12Z] //:gcs_actor_manager_export_event_test FAILED TO BUILD
  | [2024-12-29T12:19:12Z] //:gcs_actor_manager_test FAILED TO BUILD
  | [2024-12-29T12:19:12Z] //:gcs_autoscaler_state_manager_test FAILED TO BUILD

do we actually use those callbacks in unit tests?

[dentiny]

do we actually use those callbacks in unit tests?

No, they're unused; I should fix the compilation in the latest commit.

[jjyao]

//python/ray/tests:test_runtime_env_working_dir_3 failure might be real (I didn't see if failed in your other PRs)

[dentiny]

//python/ray/tests:test_runtime_env_working_dir_3 failure might be real (I didn't see if failed in your other PRs)

I still think this PR is a no-op change, let me merge and run again...

[jjyao]

Consistent test failure so it might be due to this PR.