Retire Flow Cross amendment (#5562)

The FlowCross amendment is now permanently enabled, so all code branches that have this amendment disabled are removed.

src/test/app/SetAuth_test.cpp

@@ -75,7 +75,6 @@ struct SetAuth_test : public beast::unit_test::suite
     {
         using namespace jtx;
         auto const sa = supported_amendments();
-        testAuth(sa - featureFlowCross - featurePermissionedDEX);
         testAuth(sa - featurePermissionedDEX);
         testAuth(sa);
     }