Fix remote mirror validation error
Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/1062
I've removed the validation that checks whether the project import URL is the same as the project mirror URL.
This is assuming the reason for that validation was to prevent a new remote with the same URL to be added? It looks like we already prevent that as long as it has the same ref name (added relevant spec)
/cc @rdavila