net: Kill MSG_SENDPAGE_NOTLAST

Now that ->sendpage() has been removed, MSG_SENDPAGE_NOTLAST can be cleaned
up.  Things were converted to use MSG_MORE instead, but the protocol
sendpage stubs still convert MSG_SENDPAGE_NOTLAST to MSG_MORE, which is now
unnecessary.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: linux-afs@lists.infradead.org
cc: mptcp@lists.linux.dev
cc: rds-devel@oss.oracle.com
cc: tipc-discussion@lists.sourceforge.net
cc: virtualization@lists.linux-foundation.org
Link: https://lore.kernel.org/r/20230623225513.2732256-17-dhowells@redhat.com


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