Draft: Delete groups if they don't exist in the color scheme

Since 0d0bca58 we use the lnf theme for defaults. If a lnf feel is used that is not the default one kdedefaults/kdeglobals will also include the exported colors. If one now selects the a color scheme without header colors we would mark the entries in the relevant groups as deleted but not the group itself. That means that the various checks of KColorScheme::isColorSetSupported succeed in multiple places and the colors are read from the lnf defaults. This results in wrong titlebar colors in this scenario. An easy way to test this is selecting Breeze Dark as global theme and then selecting a color scheme without header colors.