Remove on-by-default gitaly_go_user_update_branch feature flag
Remove the the gitaly_go_user_update_branch feature flag that's been on by default since my 866b0849 (Turn UserUpdateBranch in Go on by default, 2021-03-23) merged in [1]. This brings us one step[2] closer to removing this entirely.
As noted in [3] the underlying Ruby code is not being removed in this commit, we'll have to wait until this change has been out for a while to avoid the race condition described in that documentation.