[nightly] Remove grpc staging test since nightly is stable

[fishbone]

Why are these changes needed?

After baking there for a while, it looks good. Remove grpc staging test for now.

Related issue number

Closes #19438

Checks

• 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 :(

[rkooo567]

Yay!