Show explanation for deleting additional certificates in message box
The showToolTip helper which made the tool tip accessible doesn't work
(or even build) with Qt 6 on Windows anymore. Therefore, we decided to
use a regular message box instead.
(cherry picked from commit 844d37f72658f6f7a81733514b734b8242ba75a3)
- GnuPG-bug-id: T7073