Remove using namespace declarations at namespace scope in headers

src/test/app/ValidatorSite_test.cpp

@@ -174,6 +174,7 @@ private:
 
         auto const sequence = 1;
         auto const version = 1;
+        using namespace std::chrono_literals;
         NetClock::time_point const expiration =
             env.timeKeeper().now() + 3600s;