powerpc/prom_init: drop PROM_BUG()

Unused, let's drop it.

Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220920122302.99195-3-david@redhat.com