Remove non-useful tooltip while loading updates
This tooltip was added to help diagnose issues with stuck backends, but has proven troublesome. This commit removes it for the following reasons:
- It did not succeed in fixing the problem in the first place because the root cause always seems to be the PackageKit backend being stuck for a moment and not issuing updates, so the tooltip doesn't change at all or even tell us which backend is stuck.
- Due to a bug, the tooltip was always shown when the cursor was hovered anywhere over the main panel, so users saw it much more than we wanted and filed many bug reports.
This commit partially reverts 2196ed49.
BUG: 432044 FIXED-IN: 5.21.5