libceph: Remove unused ceph_pagelist functions

ceph_pagelist_truncate() and ceph_pagelist_set_cursor() have been unused
since commit
39be95e9 ("ceph: ceph_pagelist_append might sleep while atomic")

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>