operations: Always enable squashing without worktrees
Starting with 007b43e5 (operations: Support worktreeless squashing for non-fast-forward merges, 2021-07-28), we suppor squashing without any worktrees in all cases. This both improves performance by ~4x and also fixes issue we've had with patching the worktree when binary files were modified.
This feature flag has been rolled out to production on August 9th, and no issues have been observed. This commit thus removes the feature flag altogether such that we never use worktrees for squashes anymore.
Changelog: performance
Changelog: bug
Closes #3713 (closed)