Replace polygons module with the one from utils

We moved it in https://github.com/alphagov/notifications-utils/pull/818/files

app/models/broadcast_message.py

@@ -1,12 +1,12 @@
 import itertools
 from datetime import datetime, timedelta
 
+from notifications_utils.polygons import Polygons
 from notifications_utils.template import BroadcastPreviewTemplate
 from orderedset import OrderedSet
 from werkzeug.utils import cached_property
 
 from app.broadcast_areas import CustomBroadcastAreas, broadcast_area_libraries
-from app.broadcast_areas.polygons import Polygons
 from app.formatters import round_to_significant_figures
 from app.models import JSONModel, ModelList
 from app.models.user import User