Stop referring to service.postage

All templates have postage now and we will be getting rid of
`service.postage`.

app/utils.py

@@ -332,7 +332,7 @@ def get_template(
                 image_url=letter_preview_url,
                 page_count=int(page_count),
                 contact_block=template['reply_to_text'],
-                postage=template.get('postage') or service.postage,
+                postage=template['postage'],
             )
         else:
             return LetterPreviewTemplate(