powerpc: Remove last traces of POWER4_ONLY

The Kconfig symbol POWER4_ONLY got removed in commit
694caf02 ("powerpc: Remove
CONFIG_POWER4_ONLY"). Remove its last traces.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>