Remove using namespace declarations at namespace scope in headers

src/test/app/RCLValidations_test.cpp

@@ -101,6 +101,7 @@ class RCLValidations_test : public beast::unit_test::suite
         std::vector<std::shared_ptr<Ledger const>> altHistory(
             history.begin(), history.begin() + diverge);
         // advance clock to get new ledgers
+        using namespace std::chrono_literals;
         env.timeKeeper().set(env.timeKeeper().now() + 1200s);
         prev = altHistory.back();
         bool forceHash = true;