netfs: Remove netfs_extract_user_iter()

Remove netfs_extract_user_iter() as it has been replaced with
netfs_extract_iter().

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara <pc@manguebit.org>
cc: Matthew Wilcox <willy@infradead.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org