Remove feature flag for #30974

[frankmcsherry]

Remove the defensive feature flag for #30974, which seems to have landed without incident.

Motivation

Tips for reviewer

Checklist

• This PR has adequate test coverage / QA involvement has been duly considered. (trigger-ci for additional test/nightly runs)
• This PR has an associated up-to-date design doc, is a design doc (template), or is sufficiently small to not require a design.
• If this PR evolves an existing $T ⇔ Proto$T mapping (possibly in a backwards-incompatible way), then it is tagged with a T-proto label.
• If this PR will require changes to cloud orchestration or tests, there is a companion cloud PR to account for those changes that is tagged with the release-blocker label (example).
• If this PR includes major user-facing behavior changes, I have pinged the relevant PM to schedule a changelog post.

[ggevay]

This will be rolled out next week. (The PR was merged on Friday, and we do the release cut on Thursday evening/night.)

[ggevay]

The feature has been rolled out yesterday, so this PR can go in now.