ceph: remove unused functions in ceph_frag.h

These functions were introduced in commit 3d14c5d2 ("ceph: factor
out libceph from Ceph file system"). Howover, there's no user of
these functions since then, so remove them for simplicity.

Signed-off-by: Yaowei Bai <baiyaowei@cmss.chinamobile.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>