dm: remove queue next_ordered workaround for barriers

dm: remove queue next_ordered workaround for barriers

This patch removes DM's bio-based vs request-based conditional setting
of next_ordered.  For bio-based DM the next_ordered check is no longer a
concern (as that check is now in the __make_request path).  For
request-based DM the default of QUEUE_ORDERED_NONE is now appropriate.

bio-based DM was changed to work-around the previously misplaced
next_ordered check with this commit:
865c62cd8a4b886fe0bb1cf20187bc58a9c94e13

request-based DM does not yet support barriers but reacted to the above
bio-based DM change with this commit:
eaf2832ad421360e435e43aa6330748f52a79727

The above changes are no longer needed given Neil Brown's recent fix to
put the next_ordered check in the __make_request path:
3d2a3cd5739df4da1514b0eb994c013c235f3727

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: Jun'ichi Nomura <j-nomura@ce.jp.nec.com>
Cc: NeilBrown <neilb@suse.de>
Acked-by: Kiyoshi Ueda <k-ueda@ct.jp.nec.com>
Acked-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Alasdair G Kergon <agk@redhat.com>