powerpc/boot: Remove addRamdisk.c since it is now unused

It was used in the dim distant past for adding initrds to images
for legacy iSeries, but it's not even used for that now that we
have initramfs.  So remove it.

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