ceph: remove unused vxattr length helpers

ceph_listxattr() now calculates the length of vxattrs dynamically, so
these helpers, which incorrectly ignore vxattr.exists_cb(), can be
removed.

Signed-off-by: David Disseldorp <ddiss@suse.de>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>