Stop publishing England UTLA confirmed case numbers since they don't include Pillar 2 tests

[tomwhite]

Pillar 2 includes as many as 90% of new cases (https://twitter.com/jburnmurdoch/status/1277961740697796620) so it's misleading to publish England UTLA numbers as if they are the complete picture.

This has been discussed before (e.g. #62, #66, #52), however it has become a bigger problem over time. I added a prominent warning to the README, but downstream users may not see that.