Remove _xmppconnect support

It has always been vulnerable to MITM attacks when it is not used with DNSSEC,
and has been removed from XEP-0156 because of that. We have been issued
CVE-2022-26491 for this issue.

More discussion can be found at
https://mail.jabber.org/pipermail/standards/2022-February/038759.html.

Testing Done:
Compiled

Reviewed at https://reviews.imfreedom.org/r/1357/