Revert "Remove polygonItems from their Stroke when added to a StrokesGroup"

This reverts commit 177571ca79cbd92f6544c80ce36ded88ce64fa7a.

src/domain/UBGraphicsPolygonItem.cpp

@@ -114,9 +114,6 @@ UBGraphicsPolygonItem::~UBGraphicsPolygonItem()
 
 void UBGraphicsPolygonItem::setStrokesGroup(UBGraphicsStrokesGroup *group)
 {
-    if (mStroke)
-        mStroke->remove(this);
-
     mpGroup = group;
 }