Remove robots.txt

Google’s documentation says:

> robots.txt is not a mechanism for keeping a web page out of Google. To
> keep a web page out of Google, you should use noindex directives

We’ve implemented a noindex directive now, so we don’t need to serve
robots.txt any more.

tests/app/main/views/test_index.py

@@ -63,18 +63,8 @@ def test_logged_in_user_redirects_to_choose_account(
     )
 
 
-def test_robots(client):
-    assert url_for('main.robots') == '/robots.txt'
-    response = client.get(url_for('main.robots'))
-    assert response.headers['Content-Type'] == 'text/plain'
-    assert response.status_code == 200
-    assert response.get_data(as_text=True) == (
-        'User-agent: *\n'
-        'Disallow: /sign-in\n'
-        'Disallow: /support\n'
-        'Disallow: /support/\n'
-        'Disallow: /register\n'
-    )
+def test_robots(client_request):
+    client_request.get_url('/robots.txt', _expected_status=404)
 
 
 @pytest.mark.parametrize('endpoint, kwargs', (