Remove all our Statsd integration.

A large proportion of the stats we're sending are misleading and/or
useless, so let's remove them and replace them with useful stats if and
when we have a well-understood need for them.
Remove the `statsd-ruby` gem.

We can reinstate it when we need it.
Merge pull request #190 from alphagov/kill-statsd

Remove all our Statsd integration.