Remove Sizewell from the demo areas library

We added this for the 25 May demo. We’re unlikely to need it again.

We’re also unlikely to need this library again, so this commit removes
the source and the code that creates it.

tests/app/broadcast_areas/test_broadcast_area.py

@@ -27,11 +27,6 @@ def test_loads_libraries():
             'Countries',
             False,
         ),
-        (
-            'demo',
-            'Demo areas',
-            False,
-        ),
         (
             'wd20-lad20-ctyua19',
             'Local authorities',
@@ -149,10 +144,9 @@ def test_repository_has_all_libraries():
     repo = BroadcastAreasRepository()
     libraries = repo.get_libraries()
 
-    assert len(libraries) == 4
+    assert len(libraries) == 3
     assert [
         ('Countries', 'country'),
-        ('Demo areas', 'demo area'),
         ('Test areas', 'test area'),
         ('Local authorities', 'local authority'),
     ] == [(name, name_singular) for _, name, name_singular, _is_group in libraries]