main: Instantiate EndSessionDialog

Using an exported `init()` function to create the object is an odd
pattern, and not having the object referenced anywhere makes it harder
to access for debugging or extensions.

Just export the `EndSessionDialog` class and instantiate it like we
do for other objects.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2997>

js/ui/endSessionDialog.js

@@ -223,14 +223,7 @@ function _setCheckBoxLabel(checkBox, text) {
     }
 }
 
-export function init() {
-    // This always returns the same singleton object
-    // By instantiating it initially, we register the
-    // bus object, etc.
-    new EndSessionDialog();
-}
-
-const EndSessionDialog = GObject.registerClass(
+export const EndSessionDialog = GObject.registerClass(
 class EndSessionDialog extends ModalDialog.ModalDialog {
     _init() {
         super._init({