Remove chat feature

This was originally contributed upstream by Stanford, circa 2013.

We neither use nor support this feature in its current implementation,
and in fact, we may never have used this production. Until recently, we
had additional chat/Jabber code [1] (in the form of a Jabber djangoapp in
LMS); context there suggests this feature may have never been more than
a prototype. The original author is no longer on the team, so I can't directly
confirm this on our end.

Do you use this feature?

Stanford had already abandoned this Jabber-backed chat implementation,
in favor of an IRC backend, by the time I joined the team in early 2014.

[1] dbe52a6b13

common/test/acceptance/pages/studio/settings_advanced.py

@@ -209,7 +209,6 @@ class AdvancedSettingsPage(CoursePage):
             'annotation_token_secret',
             'showanswer',
             'show_calculator',
-            'show_chat',
             'show_reset_button',
             'static_asset_path',
             'text_customization',