Remove references to the toolbox plugin from the themes.json

Some other components were (incorrectly) using these theme variables.
Replaced them by another theme variable with the same value.

CURA 8588

plugins/DigitalLibrary/resources/qml/ProjectSummaryCard.qml

@@ -44,7 +44,7 @@ Cura.RoundedRectangle
         {
             id: projectImage
             anchors.verticalCenter: parent.verticalCenter
-            width: UM.Theme.getSize("toolbox_thumbnail_small").width
+            width: UM.Theme.getSize("card_icon").width
             height: Math.round(width * 3/4)
             sourceSize.width: width
             sourceSize.height: height