[scn] fix failure of mittest after refresh feature scn

src/share/ob_ddl_task_executor.cpp

@@ -20,10 +20,6 @@
 using namespace oceanbase::share;
 using namespace oceanbase::common;
 
-namespace oceanbase
-{
-namespace share
-{
 ObDDLTaskQueue::ObDDLTaskQueue()
   : task_list_(), task_set_(), lock_(), is_inited_(false), allocator_()
 {
@@ -339,6 +335,3 @@ int ObDDLReplicaBuilder::push_task(ObAsyncTask &task)
   }
   return ret;
 }
-
-}  // end namespace share
-}  // end namespace oceanbase