Fix exception in shutdown
The update_checker-removal patch was borked:
Traceback (most recent call last):
File "/usr/lib64/calibre/calibre/gui2/ui.py", line 904, in closeEvent
self.shutdown(write_settings=False)
File "/usr/lib64/calibre/calibre/gui2/ui.py", line 847, in shutdown
if self.update_checker:
AttributeError: 'Main' object has no attribute 'update_checker'