Future of ecmaFeatures.globalReturn

**Problem:** `globalReturn` can't be used with `sourceType:module`, and right now in ESLint we will detect this combination and set `globalReturn` to `false`. Arguably, this behavior is undesirable because ESLint is silently fixing a problem rather than letting the user know that it happened.

We recently just implemented `sourceType: commonjs`, which effectively makes `ecmaFeatures.globalReturn` obsolete, so there are several options we could pursue going forward:

1. We could remove `ecmaFeatures.globalReturn` altogether. I think this is the cleanest solution now that we have `sourceType: commonjs`, however, we would probably want to throw an error if `ecmaFeatures.globalReturn` is specified to let people know that it has been removed.
2. We could keep `ecmaFeatures.globalReturn` but throw an error if it is used with `sourceType: module`. This error would bubble up to ESLint and to the user, allowing them to fix their configuration.

In either case, we could remove the logic from ESLint completely.

Thoughts?