libceph: drop CEPH_DEFINE_SHOW_FUNC

Although CEPH_DEFINE_SHOW_FUNC is much older, it now duplicates
DEFINE_SHOW_ATTRIBUTE from linux/seq_file.h.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>