Remove the counties and unitary authorities library

It’s been superceded by the ‘Local’ library (formerly ‘Electoral wards
in the United Kingdom’).

The latter is better because:
- it’s covers all 4 nations, not just England and Wales
- it has electoral wards as well as local authorities which group them,
  so there’s more flexibility when choosing an area to broadcast to

app/broadcast_areas/create-broadcast-areas-db.py

@@ -70,7 +70,6 @@ repo.create_tables()
 
 simple_datasets = [
     ("Countries", "ctry19cd", "ctry19nm"),
-    ("Counties and Unitary Authorities in England and Wales", "ctyua16cd", "ctyua16nm"),
 ]
 for dataset_name, id_field, name_field in simple_datasets:
     filepath = package_path / "{}.geojson".format(dataset_name)