splice: Remove some now-unused bits

Remove some code that's no longer used as the ->confirm() op is no longer
used and pages spliced in from the pagecache and process VM are now
pre-stolen or copied.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Matthew Wilcox <willy@infradead.org>
cc: Dave Chinner <david@fromorbit.com>
cc: Christoph Hellwig <hch@lst.de>
cc: Jens Axboe <axboe@kernel.dk>
cc: linux-fsdevel@vger.kernel.org