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:

1. 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.
2. 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