launch-new-instances: Drop obsolete signals and callbacks

The 'launching' signal and this._onActivateOverride callback were
removed from gnome-shell in 7ecb5af587af7ed892c6cfc5af0858a2acb04905
and 7ecb5af587af7ed892c6cfc5af0858a2acb04905 respectively.

https://bugzilla.gnome.org/show_bug.cgi?id=721864

extensions/launch-new-instance/extension.js

@@ -5,14 +5,7 @@ var _onActivateOriginal = null;
 var _activateResultOriginal = null;
 
 function _onActivate(event) {
-
-  this.emit('launching');
-
-  if (this._onActivateOverride) {
-    this._onActivateOverride(event);
-  } else {
-    this.app.open_new_window(-1);
-  }
+  this.app.open_new_window(-1);
   Main.overview.hide();
 }