Remove Google Talk from the XMPP plugin.

Google Talk has long been on the deprecated list and with Hangouts getting
retired in June, it seems fitting that we finally remove this code.

Testing Done:
* Compiled locally
* Ran the unit tests
* Connected an XMPP account
* Had an audio call with Pidgin 2
* Had a video call with Pidgin 2

Reviewed at https://reviews.imfreedom.org/r/435/