Use template_type property directly from API

It’s included in the job/upload response now[1]. So we don’t need to
fetch the template every time we want to access it.

1. https://github.com/alphagov/notifications-api/pull/2728

app/models/job.py

@@ -30,6 +30,7 @@ class Job(JSONModel):
         'processing_started',
         'notification_count',
         'created_by',
+        'template_type',
     }
 
     @classmethod
@@ -132,10 +133,6 @@ class Job(JSONModel):
             version=self.template_version,
         )['data']
 
-    @property
-    def template_type(self):
-        return self.template['template_type']
-
     @property
     def percentage_complete(self):
         return self.notifications_requested / self.notification_count * 100