gitaly-git2go/commit: Revert validation for existing target paths

We have started to observe another regression in snippet service tests when moving files via the UserCommit RPC call. Given that I don't fully understand the failure mode right now and that the regression is keeping us from upgrading our Gitaly version, I'm reverting the change that is causing the regression even though it does feel like the right thing to do. We should investigate why tests are failing in Rails after tests are rolling again -- chances are high that it's simply a weird test compared to a real regression.

Revert 07abb8a2 (Merge branch 'xx/fix-create-file-with-path-already-exists' into 'master', 2021-10-18) to unblock ourselves.

Fixes #3864 (closed)