Remove the upload_letters permission

Every service has it now, and we haven’t had any services ask to toggle
it off again.

tests/app/main/views/service_settings/test_service_setting_permissions.py

@@ -119,9 +119,6 @@ def test_service_set_permission(
     ({'restricted': False}, '.service_switch_live', {}, 'Live On Change service status'),
     ({'permissions': ['sms']}, '.service_set_inbound_number', {},
         'Receive inbound SMS Off Change your settings for Receive inbound SMS'),
-    ({'permissions': ['letter']},
-     '.service_set_permission', {'permission': 'upload_letters'},
-        'Uploading letters Off Change your settings for Uploading letters'),
     ({'permissions': ['letter']},
      '.service_set_permission', {'permission': 'international_letters'},
         'Send international letters Off Change your settings for Send international letters'),

tests/app/main/views/uploads/test_upload_letter.py

@@ -629,8 +629,7 @@ def test_send_uploaded_letter_sends_letter_and_redirects_to_notification_page(
 
 @pytest.mark.parametrize('permissions', [
     ['email'],
-    ['letter'],
-    ['upload_letters'],
+    ['sms'],
 ])
 def test_send_uploaded_letter_when_service_does_not_have_correct_permissions(
     mocker,