[POWERPC] Remove unused platform_machine_check()

Remove leftover cruft from ARCH=ppc.

There are no users of platform_machine_check() in ARCH=powerpc, and none
should be added (they should use ppc_md.machine_check_handler instead).

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>