powerpc: Remove ppc64_boot_msg

ppc64_boot_msg is meant to be a boot debug aid, but
is only used in one spot. Get rid of it, and save
ourseleves a couple of lines in the kernel log
buffer.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>