kbuild: sh: remove unused install script

The sh arch has a install.sh script, but no Makefile actually calls it.
Remove it to keep anyone from accidentally calling it in the future.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>