skd: rip out bio path

The skd driver has a selectable rq or bio based queueing model.
For 3.14, we want to turn this into a single blk-mq interface
instead. With the immutable biovecs being merged in 3.13, the
bio model would need patches to even work. So rip it out, with
a conversion pending for blk-mq in the next release.
Signed-off-by: Jens Axboe <axboe@kernel.dk>