Make KStandardGuiItem::discard match QDialogButtonBox::Discard

Summary:
Make KStandardGuiItem::discard match QDialogButtonBox::Discard, which is being changed in D8029

Completeness fix for 385158

Reviewers: davidedmundson, Frameworks, cfeck

Reviewed By: cfeck

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D8036