UI Freeze Exception: update-notifier removal of Ubuntu Pro notification

[Robert Ancell]

Hi,

Due to backend delay, we want to disable the Ubuntu Pro notification. This
feature will be re-added at a future time.

The notification will no longer display, but the code will remain so
strings will still be available for translation.
See https://bugs.launchpad.net/ubuntu/+source/update-notifier/+bug/1965996
for more details.

Thanks,
--Robert