Revert "Correct inabillity for appservice-discord to connect"
This reverts commit 673e19f830.

While certain things do work even with such a local URL, sending
messages leads to an error like this:

> [DiscordBot] verbose: DiscordAPIError: Invalid Form Body
> avatar_url: Not a well formed URL.

Fixes https://github.com/Half-Shot/matrix-appservice-discord/issues/649

The sample configuration file for appservice-discord
c29cfc72f5/config/config.sample.yaml (L8)
explicitly says that we need a public URL.