remove the mg_disk driver

This drivers was added in 2008, but as far as a I can tell we never had a
single platform that actually registered resources for the platform driver.

It's also been unmaintained for a long time and apparently has a ATA mode
that can be driven using the IDE/libata subsystem.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@fb.com>