Revert "ContextMenu: Allow plugins submenus to react to Shift modifiers"

This reverts commit ce3c2435.

The apidox of setParentWidget says:

    Set the parent widget for any dialogs being shown.
    This should normally be your mainwindow, not a popup menu, so that it still exists even after the popup is closed
    (e.g. error message from KRun) and so that QAction::setStatusTip can find a statusbar, too.

CCBUG: 425997
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