removed duplicated assertions from String.unit.hx

tests/unit/src/unitstd/String.unit.hx

@@ -152,9 +152,6 @@ s.substring(2, -1) == "xf";
 s.substring(0) == "xfooxfooxxbarxbarxx";
 s.substring(1) == "fooxfooxxbarxbarxx";
 s.substring(2) == "ooxfooxxbarxbarxx";
-s.substring(0, -1) == "";
-s.substring(1, -1) == "x";
-s.substring(2, -1) == "xf";
 s.substring(20, 0) == "xfooxfooxxbarxbarxx";
 s.substring(0, 100) == "xfooxfooxxbarxbarxx";
 s.substring(100, 120) == "";