Remove _onKeyPress() function in Notification

Its usage was removed in commit 4dd4c9f99f -
"Use more actor.grab_key_focus() and less stage.connect('key-press-event')".

js/ui/messageTray.js

@@ -706,15 +706,6 @@ Notification.prototype = {
             this.destroy();
     },
 
-    _onKeyPress: function(actor, event) {
-        let symbol = event.get_key_symbol();
-        if (symbol == Clutter.Escape) {
-            Main.messageTray.escapeTray();
-            return true;
-        }
-        return false;
-    },
-
     ungrabFocus: function() {
         if (!this._hasFocus)
             return;