powerpc: remove unused zInstall target from arch/powerpc/boot/Makefile

Commit c913e5f9 ("powerpc/boot: Don't install zImage.* from make
install") added the zInstall target to arch/powerpc/boot/Makefile,
but you cannot use it since the corresponding hook is missing in
arch/powerpc/Makefile.

It has never worked since its addition. Nobody has complained about
it for 7 years, which means this code was unneeded.

With this removal, the install.sh will be passed in with 4 parameters.
Simplify the shell script.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210729141937.445051-1-masahiroy@kernel.org