Remove bitbucket_server_convert_mentions_to_users feature flag

What does this MR do and why?

Removes the bitbucket_server_convert_mentions_to_users feature flag that has been globally enabled on .com since 2024-01-31.

Rollout issue: [Feature flag] Rollout of `bitbucket_server_con... (#434453 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Relates to #434453 (closed)