Use real letter_rates in tests

Added the letter_rate table to the list of tables which does not get
deleted after each test run and changed the tests to use the real letter
rates.

Also removed the letter rate DAO since this was only being used in
tests, so was no longer needed.

tests/app/db.py

@@ -32,7 +32,6 @@ from app.models import (
     SMS_TYPE,
     KEY_TYPE_NORMAL,
     AnnualBilling,
-    LetterRate,
     InvitedOrganisationUser,
     FactBilling,
     FactNotificationStatus,
@@ -475,28 +474,6 @@ def create_annual_billing(
     return annual_billing
 
 
-def create_letter_rate(
-        start_date=datetime(2017, 1, 1, 00, 00, 00),
-        end_date=None,
-        sheet_count=1,
-        rate=0.31,
-        crown=True,
-        post_class='second'
-):
-    rate = LetterRate(
-        start_date=start_date,
-        end_date=end_date,
-        sheet_count=sheet_count,
-        rate=rate,
-        crown=crown,
-        post_class=post_class
-    )
-    db.session.add(rate)
-    db.session.commit()
-
-    return rate
-
-
 def create_organisation(name='test_org_1', active=True):
     data = {
         'name': name,