Conpherence - fix durable column message duplicating bug

Summary: Fixes T6713. Though I've said that before. =D Looks like this handler wasn't upgraded earlier and was still updating the DOM; removing the DOM updating code and let the central spot handle everything and this works fine.

Test Plan: open up two browsers with durable column on same room. send messages in browser a and observe 1 copy of each message showing up in browser b. send messages in browswer b and observe one copy in browser a. browser a was chrome and browser b was firefox.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T6713

Differential Revision: https://secure.phabricator.com/D13214