drivers/block: remove the umem driver

This removes the driver on the premise that it has been unused for a long
time. This is a better approach compared to changing untestable code
nobody cares about in the first place. Similarly, the umem.com website now
shows a mere Godaddy parking add.

Acked-by: NeilBrown <neilb@suse.de>
Suggested-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>