kbuild: simplify GCC_PLUGINS enablement in dummy-tools/gcc

commit f4c3b83b upstream.

With commit 1e860048

 ("gcc-plugins: simplify GCC plugin-dev
capability test") applied, this hunk can be way simplified because
now scripts/gcc-plugins/Kconfig only checks plugin-version.h

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>