Stop guessing notification type

Before the search term was either:
- an email address (or partial email address)
- a phone number (or partial phone number)

Now it can also be:
- a reference (or partial reference)

We can take a pretty good guess, by looking at the search term, whether
the thing the user is searching by email address or phone number. This
helps us:
- only show relevant notifications
- normalise the search term to give the best chance of matching what we
  store in the `normalised_to` field

However we can’t look at a search term and guess whether it’s a
reference, because a reference could take any format. Therefore if the
user hasn’t told us what kind of thing their search term is, we should
stop trying to guess.

app/dao/notifications_dao.py

@@ -1,5 +1,4 @@
 import functools
-import string
 from itertools import groupby
 from operator import attrgetter
 from datetime import (
@@ -570,8 +569,6 @@ def dao_update_notifications_by_reference(references, update_dict):
 
 @statsd(namespace="dao")
 def dao_get_notifications_by_recipient_or_reference(service_id, search_term, notification_type=None, statuses=None):
-    if notification_type is None:
-        notification_type = guess_notification_type(search_term)
 
     if notification_type == SMS_TYPE:
         normalised = try_validate_and_format_phone_number(search_term)
@@ -587,9 +584,12 @@ def dao_get_notifications_by_recipient_or_reference(service_id, search_term, not
         except InvalidEmailError:
             normalised = search_term.lower()
 
-    else:
+    elif notification_type == LETTER_TYPE:
         raise InvalidRequest("Only email and SMS can use search by recipient", 400)
 
+    else:
+        normalised = search_term.lower()
+
     normalised = escape_special_characters(normalised)
     search_term = escape_special_characters(search_term)
 
@@ -765,13 +765,6 @@ def dao_precompiled_letters_still_pending_virus_check():
     return notifications
 
 
-def guess_notification_type(search_term):
-    if set(search_term) & set(string.ascii_letters + '@'):
-        return EMAIL_TYPE
-    else:
-        return SMS_TYPE
-
-
 def _duplicate_update_warning(notification, status):
     current_app.logger.info(
         (