Fix inability to send (Matrix -> Discord) messages via appservice-discord

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.

roles/matrix-bridge-appservice-discord/defaults/main.yml

@@ -37,7 +37,7 @@ matrix_appservice_discord_appservice_url: 'http://matrix-appservice-discord:9005
 
 matrix_appservice_discord_bridge_domain: "{{ matrix_domain }}"
 # As of right now, the homeserver URL must be a public URL. See below.
-matrix_appservice_discord_bridge_homeserverUrl: "http://matrix-synapse:8008"
+matrix_appservice_discord_bridge_homeserverUrl: "{{ matrix_homeserver_url }}"
 matrix_appservice_discord_bridge_disablePresence: false
 matrix_appservice_discord_bridge_enableSelfServiceBridging: false