plasmaquick/containmentitem: remove desktoptoolbox support

Christoph Wolkrequested to merge
cwo/libplasma:no_more_desktoptoolbox into master

The desktop toolbox was removed with the introduction of the zoomout edit mode for Plasma 6.1, while the panel toolbox remains. ContainmentItem still tries to load both of them, but fails for the desktop and warns about it.

Instead, only load the toolboc for the panel, which simplifies the code and avoids the warning.

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