bindings: Retires [Custom=Wrap].

Since CSSValue.idl and its family were removed, we no longer need
to support [Custom=Wrap].

BUG=235436

Review URL: https://codereview.chromium.org/726553002

git-svn-id: svn://svn.chromium.org/blink/trunk@185292 bbb929c8-8fbe-4397-9dbb-9b2b20218538