Remove platform admin letter/international buttons

Users can choose whether to have these features on or off now. And
a platform admin can still use the same controls to switch the features
on or off on behalf of a user. There’s no need for the separate buttons.

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

@@ -31,9 +31,6 @@ def get_service_settings_page(
     ({'permissions': ['email']}, '.service_switch_can_send_email', {}, 'Stop sending emails'),
     ({'permissions': []}, '.service_switch_can_send_email', {}, 'Allow to send emails'),
 
-    ({'permissions': ['letter']}, '.service_switch_can_send_letters', {}, 'Stop sending letters'),
-    ({'permissions': []}, '.service_switch_can_send_letters', {}, 'Allow to send letters'),
-
     ({'permissions': ['sms']}, '.service_switch_can_send_sms', {}, 'Stop sending sms'),
     ({'permissions': []}, '.service_switch_can_send_sms', {}, 'Allow to send sms'),
 

tests/app/main/views/test_service_settings.py

@@ -530,8 +530,6 @@ def test_route_permissions(
     'main.service_request_to_go_live',
     'main.service_switch_live',
     'main.service_switch_research_mode',
-    'main.service_switch_can_send_letters',
-    'main.service_switch_can_send_international_sms',
     'main.archive_service',
 ])
 def test_route_invalid_permissions(
@@ -584,8 +582,6 @@ def test_route_for_platform_admin(
 @pytest.mark.parametrize('route', [
     'main.service_switch_live',
     'main.service_switch_research_mode',
-    'main.service_switch_can_send_letters',
-    'main.service_switch_can_send_international_sms',
 ])
 def test_route_for_platform_admin_update_service(
         mocker,