More cleanup now self.window is gone

No need for self.main_window in gui object.
No need for hooks "init_qt" and "close_main_window".

lib/plugins.py

@@ -149,9 +149,6 @@ def _run_hook(name, always, *args):
     for p, f in f_list:
         if name == 'load_wallet':
             p.wallet = args[0]
-        if name == 'init_qt':
-            gui = args[0]
-            p.window = gui.main_window
         if always or p.is_enabled():
             try:
                 r = f(*args)