Refactor use ServiceOnOffForm

We no longer need a custom form object here – ServiceOnOffForm does the
same job of turning strings to booleans and back again.

app/main/forms.py

@@ -1719,16 +1719,6 @@ class ServiceDeliveryStatusCallbackForm(CallbackForm):
     )
 
 
-class InternationalSMSForm(StripWhitespaceForm):
-    enabled = RadioField(
-        'Send text messages to international phone numbers',
-        choices=[
-            ('on', 'On'),
-            ('off', 'Off'),
-        ],
-    )
-
-
 class SMSPrefixForm(StripWhitespaceForm):
     enabled = RadioField(
         '',