lightnvm: remove lower page tables

The lower page table is unused. All page tables reported by 1.2
devices are all reporting a sequential 1:1 page mapping. This is
also not used going forward with the 2.0 revision.

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