lightnvm: remove mlc pairs structure

The known implementations of the 1.2 specification, and upcoming 2.0
implementation all expose a sequential list of pages to write.
Remove the data structure, as it is no longer needed.

Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>