powerpc: Remove no longer used ppc_md.idle_loop()

The last user of ppc_md.idle_loop() was removed when we dropped the
legacy iSeries code, in commit 8ee3e0d6

.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>