[es6] Remove harmony-classes flag

test/js-perf-test/JSTests.json

 {
   "name": "JSTests",
   "run_count": 5,
   "run_count_android_arm": 3,
   "run_count_android_arm64": 3,
   "timeout": 120,
   "units": "score",
   "total": true,
   "resources": ["base.js"],
   "tests": [
@@ skipping 10 matching lines @@
         {"name": "Call"},
         {"name": "CallMethod"},
         {"name": "CallNew"}
       ]
     },
     {
       "name": "Classes",
       "path": ["Classes"],
       "main": "run.js",
       "resources": ["super.js", "default-constructor.js"],
-      "flags": ["--harmony-classes"],
       "results_regexp": "^%s\\-Classes\\(Score\\): (.+)$",
       "tests": [
         {"name": "Super"},
         {"name": "DefaultConstructor"}
       ]
     },
     {
       "name": "Collections",
       "path": ["Collections"],
       "main": "run.js",
@@ skipping 80 matching lines @@
       "path": ["Exceptions"],
       "main": "run.js",
       "resources": ["try-catch.js"],
       "results_regexp": "^%s\\-Exceptions\\(Score\\): (.+)$",
       "tests": [
         {"name": "Try-Catch"}
       ]
     }
   ]
 }