Remove duplicate property test

In ES6, these are now allowed in both strict and sloppy mode.

These tests will fail when v8:3498 lands and gets rolled.

BUG=403353
R=dpranke@chromium.org

Review URL: https://codereview.chromium.org/467903003

git-svn-id: svn://svn.chromium.org/blink/trunk@180197 bbb929c8-8fbe-4397-9dbb-9b2b20218538