block: don't set REQ_NOMERGE unnecessarily

RQ_NOMERGE_FLAGS already clears defines which REQ flags aren't
mergeable.  There is no reason to specify it superflously.  It only
adds to confusion.  Don't set REQ_NOMERGE for barriers and requests
with specific queueing directive.  REQ_NOMERGE is now exclusively used
by the merging code.

[ Impact: cleanup ]

Signed-off-by: Tejun Heo <tj@kernel.org>