Delete js-test-post.js.

There turn out to be three copies of js-test-pre/post in the tree.
This deletes the one used by all but ~100 tests.

TBR=eae@chromium.org

Review URL: https://codereview.chromium.org/48903019

git-svn-id: svn://svn.chromium.org/blink/trunk@161210 bbb929c8-8fbe-4397-9dbb-9b2b20218538