powerpc/numa: remove unreachable topology update code

Since the topology_updates_enabled flag is now always false, remove it
and the code which has become unreachable. This is the minimum change
that prevents 'defined but unused' warnings emitted by the compiler
after stubbing out the start/stop_topology_updates() functions.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200612051238.1007764-5-nathanl@linux.ibm.com