Remove duplicate VoidCallback classes now that no callbacks are RefCounted

BUG=323681
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/105633003

git-svn-id: svn://svn.chromium.org/blink/trunk@163283 bbb929c8-8fbe-4397-9dbb-9b2b20218538