console: apply null as `this` for util.format

Util.format is just a stateless function. Apply current console
as `this` is unnecessary.

PR-URL: https://github.com/nodejs/node/pull/5222


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>