libceph: Remove unused pagevec functions

ceph_copy_user_to_page_vector() has been unused since 2013's commit
e8344e66 ("ceph: Implement writev/pwritev for sync operation.")

ceph_copy_to_page_vector() has been unused since 2012's commit
913d2fdc ("rbd: always pass ops array to rbd_req_sync_op()")

Remove them.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>