lightnvm: remove unnecessary field from nvm_rq

Remove the wait filed in nvm_rq. It is not used anymore, as targets rely
on the functionality provided by the LightNVM subsystem when sending
sync I/O.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@kernel.dk>