Prune the email domains list

We only need domains in here which either:
- don’t belong to a single organisation (eg gov.uk)

All other domains should be stored in the database.

This PR removes domains which are now in the database.

Before
---

```sql
select domain from domain where domain in ('gov.uk', 'mod.uk', 'mil.uk', 'd
 dc-mod.org', 'gov.scot', 'parliament.scot', 'parliament.uk', 'nhs.uk', 'nhs.net', 'nhs.scot', 'police.uk', 'scotent.c
 o.uk', 'assembly.wales', 'cjsm.net', 'gov.wales', 'ac.uk', 'sch.uk', 'onevoicewales.wales', 'mtvh.co.uk', 'wmca.org.u
 k', 'suttonmail.org');
 ```

+-----------------+
| domain          |
|-----------------+
| mtvh.co.uk      |
| wmca.org.uk     |
| gov.wales       |
| gov.scot        |
| parliament.uk   |
| assembly.wales  |
| mil.uk          |
| mod.uk          |
| ddc-mod.org     |
| parliament.scot |
| scotent.co.uk   |
+-----------------+

After
---

```sql
select domain from domain where domain in ('gov.uk', 'nhs.uk', 'nhs.ne
 t', 'nhs.scot', 'police.uk', 'cjsm.net', 'ac.uk', 'sch.uk', 'onevoicewales.wales', 'suttonmail.org') ;
```

+----------+
| domain   |
|----------|
+----------+

tests/app/main/test_validators.py

@@ -80,23 +80,17 @@ def _gen_mock_field(x):
     'test@gov.uK',
     'test@test.test.gov.uk',
     'test@test.gov.uk',
-    'test@mod.uk',
-    'test@ddc-mod.org',
-    'test@test.ddc-mod.org',
-    'test@gov.scot',
-    'test@test.gov.scot',
-    'test@parliament.uk',
-    'test@gov.parliament.uk',
     'test@nhs.uk',
     'test@gov.nhs.uk',
     'test@nhs.net',
     'test@gov.nhs.net',
+    'test@nhs.scot',
     'test@police.uk',
     'test@gov.police.uk',
     'test@GOV.PoliCe.uk',
-    'test@scotent.co.uk',
-    'test@assembly.wales',
     'test@cjsm.net',
+    'test@example.ac.uk',
+    'test@example.sch.uk',
 ])
 def test_valid_list_of_white_list_email_domains(
     client,