pidgin/pidgin

Stop automatically adding/remove conversations from the conversation manager

This was done to mimic the old conversations API and shouldn't be needed going
forward. However, since we can't create conversations right now, we're probably
missing some edge cases here.

Also make sure to keep a reference around on the conversation while removing to avoid use after frees.

Testing Done:
Trained with the turtles.

Reviewed at https://reviews.imfreedom.org/r/3049/
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