Remove unused AnimatableDouble::toCSSValue

The method AnimatableDouble::toCSSValue is unused except in tests. I've
removed it and the test helper which prints an AnimatableDouble (if we
actually find the test helper useful in the future it is trivial to
re-implement).

Review URL: https://codereview.chromium.org/267713007

git-svn-id: svn://svn.chromium.org/blink/trunk@173202 bbb929c8-8fbe-4397-9dbb-9b2b20218538