Use edit-delete icon for destructive discard action

Summary:
BUG: 385158

Use edit-delete icon for buttons that execute destructive discard actions. This is what Upstream Qt originally had, but we were overriding it to use a less-appropriate icon (edit-clear)

Test Plan:
Tested in KDE Neon. An example of the change can be seen in Spectacle's main window:

Reviewers: Frameworks, VDG, dfaure, rkflx, davidedmundson, cfeck

Reviewed By: cfeck

Subscribers: cfeck, Frameworks

Differential Revision: https://phabricator.kde.org/D8029