[refactor] Delete unused test code.

Now that all the tests have been moved off using shouldRejectPromiseWithNull() these temporary helpers can be removed.

BUG=245025

Review URL: https://codereview.chromium.org/212173007

git-svn-id: svn://svn.chromium.org/blink/trunk@170263 bbb929c8-8fbe-4397-9dbb-9b2b20218538