dm rq: cleanup leftover code from recently removed q->mq_ops branching

When commit 6a23e05c ("dm: remove legacy request-based IO path")
removed some q->mq_ops branching from map_request() it left in place a
goto that was only needed if that branching (and conditional 'r'
assignment) existed.  Now that the branching is gone map_request()'s
goto can be removed too.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>