fix errors when closing the main window with browser still open

onClose() was calling closeAllWindows(), which sends a close signal
to dialogs without force=True, causing them to fire an asynchronous
save event that calls back after the collection has been closed

instead we just rely on the fact that unloadCollection() closes all
collection windows

aqt/main.py

@@ -667,7 +667,6 @@ title="%s" %s>%s</button>''' % (
         "Called from a shortcut key. Close current active window."
         aw = self.app.activeWindow()
         if not aw or aw == self or force:
-            self.app.closeAllWindows()
             self.unloadProfile(browser=False)
         else:
             aw.close()