[cleanup] remove --harmony-classes flag from mjsunit/harmony/new-target

Unknown flag warning is adding unnecessary noise to terminal during
test runs

BUG=
LOG=N
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/1236993003

Cr-Commit-Position: refs/heads/master@{#29691}

test/mjsunit/harmony/new-target.js

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// Flags: --harmony-classes --harmony-new-target --harmony-reflect
+// Flags: --harmony-new-target --harmony-reflect
 // Flags: --harmony-rest-parameters --harmony-arrow-functions