DEV: Remove unused DiscoursePlugin class (PR #9715)

This was last modified 7 years ago, I have never seen it used, and I can’t find any mention of it on Meta. I guess it is some old way to do plugins, which has since been superseded. I think it is safe to remove?

@SamSaffron / @eviltrout do you remember the context here?

GitHub

In the early days we were less sure of how the plugin API was going to look. This probably was used at some point but later ripped out and the dead code was left behind. I think we can safely remove it.