Remove redundant "multi" parameter for MMG client

This is never overridden and can't be used in practie because all
SMS clients have to use the same interface. Removing it will make
it possible to DRY-up some of the code in this method.

app/clients/sms/mmg.py

@@ -97,14 +97,14 @@ class MMGClient(SmsClient):
     def name(self):
         return 'mmg'
 
-    def send_sms(self, to, content, reference, international, multi=True, sender=None):
+    def send_sms(self, to, content, reference, international, sender=None):
         data = {
             "reqType": "BULK",
             "MSISDN": to,
             "msg": content,
             "sender": self.from_number if sender is None else sender,
             "cid": reference,
-            "multi": multi
+            "multi": True
         }
 
         start_time = monotonic()