Remove old API shim from WebIDBCallbacks.

Looks like I forgot to include a FIXME that would have reminded us this
should have had a sunset clause.

BUG=172626

Review URL: https://codereview.chromium.org/48413006

git-svn-id: svn://svn.chromium.org/blink/trunk@160751 bbb929c8-8fbe-4397-9dbb-9b2b20218538