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.

app/dao/letter_rate_dao.py

@@ -1,23 +0,0 @@
-from app import db
-from app.dao.dao_utils import transactional
-from app.models import LetterRate
-
-
-@transactional
-def dao_create_letter_rate(letter_rate):
-    db.session.add(letter_rate)
-
-
-def get_letter_rates_for_daterange(date, crown, sheet_count, post_class='second'):
-    rates = LetterRate.query.filter(
-        LetterRate.start_date <= date
-    ).filter((LetterRate.end_date == None) |  # noqa
-             (LetterRate.end_date > date)
-             ).filter(
-        LetterRate.crown == crown
-    ).filter(
-        LetterRate.sheet_count == sheet_count
-    ).filter(
-        LetterRate.post_class == post_class
-    ).all()
-    return rates