Remove django_statsd GraphiteMiddleware

This counts requests but doesn't use a .count suffix. We only aggregate
counting metrics correctly if they use a .count suffix. Rather than
changing our aggregation rules to be very django_statsd specific I have
removed this middleware. Besides, we get the same information from the
timers since they include a counter.
Merge pull request #211 from alphagov/django-statsd-fix

Remove django_statsd GraphiteMiddleware