kbuild: rpm-pkg: remove ppc64 specific image handling

This conditional was added by commit 1a0f3d42 ("kbuild: fix
make rpm for powerpc").  Its git-log explains the default kernel
image is zImage, but obviously the current arch/powerpc/Makefile
does not set KBUILD_IMAGE, so the image file is actually vmlinux.

Moreover, since commit 09549aa1

 ("deb-pkg: Remove the KBUILD_IMAGE
workaround"), all architectures are supposed to set the full path to
the image in KBUILD_IMAGE.  I see no good reason to differentiate
ppc64 from others.  Rip off the conditional.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>