powerpc: Remove check for CONFIG_SERIAL_TEXT_DEBUG

The Kconfig symbol SERIAL_TEXT_DEBUG was removed from
arch/powerpc/Kconfig.debug in v2.6.22. (In v2.6.27 it was also removed
from arch/ppc/Kconfig.debug.) So the check for its macro has evaluated
to false for over five years now. Remove that check and the few lines
of code hidden behind it.

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