__canary__ is now in Puppet

Since https://github.gds/gds/puppet/pull/2288, the __canary__ has been in the
Nginx configuration in Puppet. We don't need this file as well as that one, as
the Nginx configuration is serving the content (and setting it to be JSON,
much like the __canary__ on GOV.UK).

Removes the file to avoid confusion. Cc: @dancarley