Remove support for old "areas" sub-field

All broadcasts with this field have now been migrated to use "ids".

This also removes a few lines that were missed in previous PRs:

- Added by mistake: fd7ebbebb0 (diff-045554136e1462693a6cbb6328b2e056a81e8b348e94575edd8f72b78c5da96eR115)
- Missed removal: ec1171f85c (diff-045554136e1462693a6cbb6328b2e056a81e8b348e94575edd8f72b78c5da96eR110)

app/models.py

@@ -2336,10 +2336,9 @@ class BroadcastMessage(db.Model):
         self._personalisation = encryption.encrypt(personalisation or {})
 
     def serialize(self):
-        # TEMPORARY: while we repurpose "areas"
         areas = dict(self.areas)
         areas["simple_polygons"] = areas.get("simple_polygons", [])
-        areas["ids"] = areas.pop("areas", areas.get("ids", []))
+        areas["ids"] = areas.get("ids", [])
 
         return {
             'id': str(self.id),