dm: rename DM_TYPE_MQ_REQUEST_BASED to DM_TYPE_REQUEST_BASED

Now that request-based DM is only using blk-mq, there is no need to
differentiate between legacy "rq" and new "mq".  We're back to a single
request-based DM -- and there was much rejoicing!

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