kbuild: drop $(objtree)/ prefix support for clean-files

I think this hack is a bad idea. arch/powerpc/boot/Makefile is the
only and last user. Let's stop doing this.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Michael Ellerman <mpe@ellerman.id.au> (powerpc)