pcbnew: fix Duplicate assertion

Do not assert that the item to be duplicated can be duplicated, just don't
duplicate it. The user can trigger a Duplicate on items that aren't
duplicatable, like field texts - he'll figure out that they can't be duplicated
when they aren't, no need to deliver a scary warning and risk an abort.

TODO: Don't pick up the duplicated selection if none of the items in it were
actually duplicated, it's misleading.

pcbnew/class_board.cpp

@@ -2708,8 +2708,7 @@ BOARD_ITEM* BOARD::DuplicateAndAddItem( const BOARD_ITEM* aItem,
 
     default:
         // Un-handled item for duplication
-        wxASSERT_MSG( false, "Duplication not supported for items of class "
-                      + aItem->GetClass() );
+        new_item = NULL;
         break;
     }