Don't restore old canvasses on reset.

No need for this anymore as PRESERVE now works as expected.
Otherwise old canvasses are restored and do not work anymore
as the placements of the new canvasses do not apply.

simgear/props/PropertyBasedMgr.cxx

@@ -96,7 +96,7 @@ namespace simgear
     _name_elements( name_elements ),
     _element_factory( element_factory )
   {
-    _props->setAttribute(SGPropertyNode::PRESERVE, true);
+
   }
 
   //----------------------------------------------------------------------------