remove provider_rates table

this was added five years ago but never used. if we want to bring back
variable rates per client we might as well get a fresh start since a lot
has changed since then.

app/dao/provider_rates_dao.py

@@ -1,11 +0,0 @@
-from app import db
-from app.dao.dao_utils import autocommit
-from app.models import ProviderDetails, ProviderRates
-
-
-@autocommit
-def create_provider_rates(provider_identifier, valid_from, rate):
-    provider = ProviderDetails.query.filter_by(identifier=provider_identifier).one()
-
-    provider_rates = ProviderRates(provider_id=provider.id, valid_from=valid_from, rate=rate)
-    db.session.add(provider_rates)