Allow differing relative paths in ReplicateRepository (15.6)

ReplicateRepository checks that the relative paths of the source and the target repositories are the same. This is more of a sanity check than a requirement. With Prafect rewriting relative paths, this check fails and causes ReplicateRepository to fail with Praefect in front. Previously the tests were not running with Praefect enabled which allowed for this regression to slip through. This commit removes the check which allows the RPC to work with Praefect enabled. The target repository has a rewritten relative path as it's where the Gitaly will store the replicated repository. The source repository's path is not rewritten by Praefect as Gitaly fetches it by going through the source storage. If the source storage is hosted by a Praefect, the relative path would be rewritten at that point by Praefect.

Backport of !5076 (merged)