Clean up MehrSchulferienWeb.FederalStateController.show and county_show

[wintermeyer]

Because the performance of https://github.com/wintermeyer/www.mehr-schulferien.de/issues/99 was so bad and the page became way too big I moved the functionality to /landkreise-und-staedte/:country_slug/:federal_state_slug (see 94ae186a284088a9fcc49950df19eb21676aad79).

@riverrun Sorry for the extra work. I totally underestimated this.

But the controller needs to be cleaned up. I'm not sure what needs to be removed where. Example (I guess this is not needed any more):

counties_with_cities =
  Enum.reduce(counties, [], fn county, acc ->
    acc ++ [{county, Locations.list_cities(county)}]
  end)

[riverrun]

Fixed in #108.