Drop broken mid-button click handling

It's been broken for years and no one complained, which probably means
no one was using the feature (middle-click to trigger toolbar actions).

While it would be trivial to fix, we probably shouldn't.
It would just make kxmlgui toolbars inconsistent with non-kxmlgui toolbars.

BUG: 383162

Differential Revision: https://phabricator.kde.org/D7153