Remove GetTor XMPP

https://tb-manual.torproject.org/downloading/

At the moment, I believe there isn't a plan to support again XMPP.

So, I'd remove this phrase:"TO USE GETTOR VIA JABBER/XMPP (JITSI, COYIM, ETC.):".

And update the docs to:

"For example, to get links for downloading Tor Browser in Chinese for Windows, send a message to gettor@torproject.org with the words "windows zh" in it."