target/ppc: Simplify cpu valid check in ppc_cpu_realize

target/ppc: Simplify cpu valid check in ppc_cpu_realize

The #if isn't necessary, because there's a suitable one inside
ppc_cpu_is_valid().  We've already filtered for suitable cpu models in the
functions that search and register them.  So by the time we get to realize
having an invalid one indicates a code error, not a user error, so an
assert() is more appropriate than error_setg().

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>