It used to be the case that GitLab created symlinks for each repository
to one copy of the Git hooks, so these ran when required. This changed
to set the hooks dynamically on Gitaly when invoking Git.

The side effect is that we didn't need all these symlinks anymore, which
Gitaly doesn't create anymore either. Now that means that the tests in
GitLab-Rails should test for it either.

Related: gitaly#1392 (comment 175619926)