Remove dummy items from group before they are deleted

Board items should not be in a group when they are deleted, but these
are dummy ones, so their group membership doesn't matter.

Fixes KICAD-7P0
Fixes KICAD-6VX
Fixes KICAD-7KK
Fixes KICAD-6VH

pcbnew/plot_board_layers.cpp

@@ -526,6 +526,8 @@ void PlotStandardLayer( BOARD* aBoard, PLOTTER* aPlotter, LSET aLayerMask,
                 // inflate/deflate a custom shape is a bit complex.
                 // so build a similar pad shape, and inflate/deflate the polygonal shape
                 PAD dummy( *pad );
+                dummy.SetParentGroup( nullptr );
+
                 SHAPE_POLY_SET shape;
                 pad->MergePrimitivesAsPolygon( &shape );