messageTray: Remove unused properties from Source

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3103>

js/ui/components/telepathyClient.js

@@ -314,7 +314,6 @@ class ChatSource extends MessageTray.Source {
 
         super._init(contact.get_alias());
 
-        this.isChat = true;
         this._pendingMessages = [];
 
         this._conn = conn;

js/ui/messageTray.js

@@ -645,12 +645,8 @@ export const Source = GObject.registerClass({
     _init(title, iconName) {
         super._init({title});
 
-        this.SOURCE_ICON_SIZE = 48;
-
         this.iconName = iconName;
 
-        this.isChat = false;
-
         this.notifications = [];
 
         this._policy = this._createPolicy();