skd: cleanup the skd_*() function block wrapping

Just call the block functions directly, don't wrap them
in skd helpers. With only one queueing model enabled, there's
no point in doing that.

Also kill the ->start_time and ->bio from the skd_request_context,
we don't use those anymore.

Signed-off-by: Jens Axboe <axboe@kernel.dk>