dm zoned: remove test for impossible REQ_OP_FLUSH conditions

The value REQ_OP_FLUSH is only used by the block code for
request-based devices.

Remove the tests for REQ_OP_FLUSH from the bio-based dm-zoned-target.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>