kbuild: remove unused cmd_none in scripts/Makefile.modinst

Commit 65ce9c38 ("kbuild: move module strip/compression code into
scripts/Makefile.modinst") added this unused code.

Perhaps, I thought cmd_none was useful for CONFIG_MODULE_COMPRESS_NONE,
but I did not use it after all.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>