remove xmpp from rest api config

An error log appeared in /var/log/jitsi/jvb.log when xmpp was used:

2020-04-08 15:58:14.603 SEVERE: [59] ComponentMain.lambda$getConnectCallable$0#293: host-unknown, host:localhost, port:5347

It might be due to a misconfiguration of prosody: https://community.jitsi.org/t/ask-xmpp-websocket-configuration/30386/4

As we do not use this, we remove xmpp for rest api config.