Remove bitbucket_cloud_convert_mentions_to_users feature flag

Carla Dragorequested to merge
442484-remove-bitbucket_cloud_convert_mentions_to_users into master

What does this MR do and why?

Removes :bitbucket_cloud_convert_mentions_to_users feature flag.

The feature flag has been enabled on production for a week. No issues have arisen as a consequence.

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Related to #442484 (closed)

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