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/Discrepancy_test.cpp

@@ -155,7 +155,6 @@ public:
     {
         using namespace test::jtx;
         auto const sa = supported_amendments();
-        testXRPDiscrepancy(sa - featureFlowCross - featurePermissionedDEX);
         testXRPDiscrepancy(sa - featureTouch - featurePermissionedDEX);
         testXRPDiscrepancy(sa - featurePermissionedDEX);
         testXRPDiscrepancy(sa);