algif: Remove hash_sendpage*()

Remove hash_sendpage*() as nothing should now call it since the rewrite of
splice_to_socket()[1].

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Link: https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=2dc334f1a63a8839b88483a3e73c0f27c9c1791c

 [1]
Signed-off-by: Jakub Kicinski <kuba@kernel.org>