Fix warnings introduced in new PartDesign

Mostly unused wariables and compare of signed & unsigned

src/Mod/Sketcher/App/SketchObject.cpp

@@ -2779,7 +2779,7 @@ int SketchObject::addExternal(App::DocumentObject *Obj, const char* SubName)
         Base::Console().Error("Internal error: counts of objects and subelements in external geometry links do not match\n");
         return -1;
     }
-    for (int i = 0  ;  i < Objects.size()  ;  i++){
+    for (size_t i = 0  ;  i < Objects.size()  ;  ++i){
         if (Objects[i] == Obj   &&   std::string(SubName) == SubElements[i]){
             Base::Console().Error("Link to %s already exists in this sketch.\n",SubName);
             return -1;