Documentation/Kbuild: Remove references to gcc-plugin.sh

gcc-plugin.sh has been removed in commit
1e860048 ("gcc-plugins: simplify GCC plugin-dev capability test").

Signed-off-by: Robert Karszniewicz <r.karszniewicz@phytec.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>