Remove log_refresh_token_hash feature flag

What does this MR do and why?

Remove the log_refresh_token_hash feature flag.

The flag has been globally enabled in production for ~5 days. The /oauth/tokens endpoint is very busy and there have been no problems. The new attribute is being logged as intended in production logs.

References

• FF issue: #573454 (closed)
• Feature issue: https://gitlab.com/gitlab-org/gitlab/-/issues/523854
• Feature MR: !207331 (merged)

How to set up and validate locally

The validation instructions from the original MR still work. refresh_token_hash is still visible in logs when this branch is checked out.

MR acceptance checklist

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Related to #573454 (closed)