RoomView: Remove another redundant unreadMessageCount reset

There's never any need to reset the unreadMessageCount in ComponentDidUpdate,
as an update can never cause there to be *fewer* unread messages. Instead we
rely on the reset in onMessageListScroll.

src/components/structures/RoomView.js

@@ -285,9 +285,6 @@ module.exports = React.createClass({
         var scrollState = this.savedScrollState;
         if (scrollState.atBottom) {
             this.scrollToBottom();
-            if (this.state.numUnreadMessages !== 0) {
-                this.setState({numUnreadMessages: 0});
-            }
         } else if (scrollState.lastDisplayedEvent) {
             this.scrollToEvent(scrollState.lastDisplayedEvent,
                                scrollState.pixelOffset);