mtd_blkdevs: don't override BLKFLSBUF

BLKFLSBUF is not supposed to actually send a flush command to the device,
but to tear down buffer cache structures.  Remove the mtd_blkdevs
implementation and just use the default semantics instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Jens Axboe <axboe@kernel.dk>