remove the lightnvm subsystem

Lightnvm supports the OCSSD 1.x and 2.0 specs which were early attempts
to produce Open Channel SSDs and never made it into the NVMe spec
proper.  They have since been superceeded by NVMe enhancements such
as ZNS support.  Remove the support per the deprecation schedule.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20210812132308.38486-1-hch@lst.de
Reviewed-by: Matias Bjørling <mb@lightnvm.io>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>