Remove using namespace declarations at namespace scope in headers

src/test/rpc/ValidatorRPC_test.cpp

@@ -269,6 +269,7 @@ public:
         //----------------------------------------------------------------------
         // Publisher list site available
         {
+            using namespace std::chrono_literals;
             NetClock::time_point const expiration{3600s};
 
             // Manage single thread io_service for server