[arcconfig] Delete subproject arcconfigs

From https://secure.phabricator.com/book/phabricator/article/arcanist_new_project/:

> An .arcconfig file is a JSON file which you check into your project's root.

I've done some experimentation, and it looks like the subproject
.arcconfigs just get ignored, as the documentation says. Given that
we're fully on the monorepo now, it's safe to remove them.

Differential Revision: https://reviews.llvm.org/D74996