lightnvm: pblk: remove size and out of bounds read check

The I/O size and capacity checks are already done by the block layer.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Reviewed-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>