Remove group association before plotting board outline in drill map

The plotter doesn't care about the grouping, and we expect the item to
have no group on deletion, so temporary items like this shouldn't be
part of one.

(Sentry issue KICAD-24Y)
(Cherry-picked from 137640ac28)

pcbnew/exporters/gen_drill_report_files.cpp

@@ -226,6 +226,7 @@ bool GENDRILL_WRITER_BASE::genDrillMapFile( const wxString& aFullFileName, PLOT_
             {
             PCB_SHAPE dummy_shape( *static_cast<PCB_SHAPE*>( item ) );
             dummy_shape.SetLayer( Dwgs_User );
+            dummy_shape.SetParentGroup( nullptr );      // Remove group association, not needed for plotting
             itemplotter.PlotPcbShape( &dummy_shape );
             }
             break;