block: get rid of unused plug->nowait flag

This was introduced to add a plug based way of signaling nowait issues,
but we have since moved on from that. Kill the old dead code, nobody is
setting it anymore.

Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>