repository: Always use two-phase voting when deleting gitattributes

In 6480bb2d (repository: Use locking two-phase voting when deleting gitattributes, 2021-12-08) we have plugged an edge case where we only performed a single transactional vote when deleting the gitattributes file so that we use proper two-phase voting. This change was rolled out to production on December 14th without any issues so far. Remove the feature flag.

Changelog: changed

Closes #3959 (closed)