ceph: Remove fs/ceph deadcode

ceph_caps_revoking() has been unused since 2017's commit
3fb99d48 ("ceph: nuke startsync op")

ceph_mdsc_open_export_target_sessions() has been unused since 2013's
commit 11df2dfb ("ceph: add imported caps when handling cap export message")

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>