Clean-up forced 404 routes once new nginx config is released

[muffinresearch]

Once the nginx configs from #10266 are rolled out to stage and prod it would be good to remove the forced 404 routes that should no longer be needed.

See https://github.com/mozilla/addons-frontend/blob/c8e567e7b6b7098368123d12fa698855c8e00d44/src/amo/routes.js#L32

[kumar303]

This will be a little confusing (for local development only) since we won't see 404s anymore. I think we can get back to 404s after #10363