ceph: Remove empty definition in header file

The real definition of ceph_acl_chmod() has been removed since
commit 4db658ea ("ceph: Fix up after semantic merge conflict"),
remain the empty definition untouched in the header files. Let's
remove the empty definition.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>