docker_registry.index: Remove local logger

This was added in d2ace65d (Naive mirroring implementation,
2014-02-06) in support of some
/v1/repositories/<path:repository>/images logging.  Those logs were
removed in bbb487a3 (Using mirroring decorator on index route,
2014-03-07), but this code was left behind.
Merge pull request #479 from wking/index-logger

docker_registry.index: Remove local logger