null: replace NULL_Q_LIGHTNVM with flag

Now that LightNVM is integrated into blk-mq and we can use
blk_lightnvm_register to create the lightnvm device. We no longer need
to special-case it in the IO path.
null: remove dead code

No need for referencing nq anymore in nullb_queue.
lightnvm: refactor max_hw_sector limitation and remove force_lightnvm

Move the max_hw_sector setup into blk-lightnvm instead of within device
drivers.

Additionally remove the force lightnvm hook used to override faulty
detection of lightnvm devices.