[tune] let placement groups be cleaned up by reconciliation

[krfricke]

Why are these changes needed?

Since we removed pending placement groups on return immediately, the trial executor staged new trial placement groups before removing them subsequently in the reconciliation process. Deferring removal until reconciliation will avoid overloading GCS with placement group requests.

cc @rkooo567

Related issue number

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 :(

[xwjiang2010]

Should we add a test that captures GCS overloading regression?

[krfricke]

Closed in favor of #18706