[POWERPC] Remove unused old code from powermac setup code

Since bootdevice never gets initialized, it's always NULL, and hence a
whole pile of code in arch/powerpc/platforms/setup.c never gets used.
(This was the code that originally was there so that the automatic
root partition selection mechanism would prefer a rootish-looking
partition on the device that OF loaded the kernel from over a similar
partition on other devices.)

This removes the unused code.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>