[tune] New persistence mode cleanup: Remove the feature flag env variable

[justinvyu]

Why are these changes needed?

This PR removes the RAY_AIR_NEW_PERSISTENCE_MODE environment variable, since the old codepath is all gone now.

Related issue number

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• 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 added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• 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
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  • This PR is not tested :(

[matthewdeng]

!