Release process: remove XMPP section.

Our current XMPP chat rooms lose this information on a weekly basis so
let's not bother with this any more.

wiki/src/contribute/release_process.mdwn

@@ -1084,11 +1084,6 @@ Remember that ticket expressions, e.g. `[[!tails_ticket 1234]]`, can
 span several lines, so finding the ones reported by the above code
 *might* be harder than `git grep "tails_ticket 1234"`.
 
-XMPP
-----
-
-Update the topic in our [[chatroom|chat]].
-
 Twitter
 -------