Remove facebook XMPP code, show error pointing at the new plugin
Facebook's oauth has been broken for months, and in the last few days
they broke plain logins too, so I just added an error message that says
this when you do "account on":
Facebook's XMPP service is gone. Try this instead:
https://wiki.bitlbee.org/HowtoFacebookMQTT
Also nuked all the oauth related code, except some parts of lib/oauth2.c
which seemed generic enough to maybe help in the future with other
not-really-compliant not-really-implementations of the not-really-oauth2
not-really-spec
