powerpc/powernv: Remove powernv RTAS support

The powernv code has some conditional support for running on bare metal
machines that have no OPAL firmware, but provide RTAS.

No released machines ever supported that, and even in the lab it was
just a transitional hack in the days when OPAL was still being
developed.

So remove the code.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>