powerpc: Remove remains of /proc/ppc_htab

commit 14cf11af ("powerpc: Merge enough to
start building in arch/powerpc.") unwired /proc/ppc_htab, and commit
917f0af9

 ("powerpc: Remove arch/ppc and
include/asm-ppc") removed the rest of the /proc/ppc_htab support, but there are
still a few references left. Kill them for good.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>