librbd: don't attempt to remove image state on orphan snapshots

librbd: don't attempt to remove image state on orphan snapshots

Despite being mirror snapshots, orphan snapshots don't have image
state: see CreateNonPrimaryRequest::write_image_state() for a similar
is_orphan() check.  Attempting to remove image state generates bogus
"failed to read image state object" and "failed to remove image state"
errors.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit cfae3f79bd0513e2753b0deb8c2624ab07cf2d1b)

Conflicts:
	src/librbd/operation/SnapshotRemoveRequest.cc [ commit
	  3a93b40721a1 ("librbd: s/boost::variant/std::variant/") not
	  in quincy ]