When we clean up refs of a merge request that is on a project
with no repository anymore, the MergeRequests::CleanupRefsService
will fail.

Before this fix, it'll throw an error and retry and will not set
the cleanup schedule as completed. When the cleanup schedule is
not marked as completed, it'll be picked up by the scheduler
again.

To fix that, we are now checking if the repository exists before
deleting refs. This way, we can still update the cleanup schedule.