MAINTAINERS: adjust GCC PLUGINS after gcc-plugin.sh removal

Commit 1e860048 ("gcc-plugins: simplify GCC plugin-dev capability test")
removed ./scripts/gcc-plugin.sh, but missed to adjust MAINTAINERS.

Hence, ./scripts/get_maintainers.pl --self-test=patterns warns:

  warning: no file matches    F:    scripts/gcc-plugin.sh

Adjust entries in GGC PLUGINS section after this file removal.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>