Remove USE_SYSTEM_GIT_FOR_FETCH

In https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4929 we added an environment variable: USE_SYSTEM_GIT_FOR_FETCH. We're relying on Git 2.16 for various performance fixes in the GCP Migration, but the upgrade to 2.16 breaks various operations on a Geo primary, so the upgrade to 2.16 been reverted.

We still want to use it on our Geo secondary, so apply a very specific EE-only workaround to permit it.

Once https://gitlab.com/gitlab-org/gitlab-ce/issues/44115 is resolved and we're using 2.16 again globally, we should remove this workaround with extreme prejudice.