Leave object pools when destroying projects
This action doesn't lean on reduplication, so a short call can me made
to the Gitaly server to have the object pool remove its remote to the
project pending deletion.
https://gitlab.com/gitlab-org/gitaly/blob/f6cd55357/internal/git/objectpool/link.go#L58
When an object pool doesn't have members, this would invalidate the need
for a pool. So when a project leaves the pool, the pool will be
destroyed on the background.
Fixes: https://gitlab.com/gitlab-org/gitaly/issues/1415
752e9c18a1