uploads: Remove display_recipient from fetched data.

This fixes a performance regression loading the Zulip homepage.

While it decreases the utility of the display of messages, it's only
so much loss (because the display recipient for PMs was totally broken
anyway).

zerver/models.py

@@ -1155,8 +1155,7 @@ class Attachment(ModelReprMixin, models.Model):
             'path_id': self.path_id,
             'messages': [{
                 'id': m.id,
-                'name': '{m.pub_date:%Y-%m-%d %H:%M} {recipient}/{m.subject}'.format(
-                    recipient=get_display_recipient(m.recipient), m=m)
+                'name': '{m.pub_date:%Y-%m-%d %H:%M}'.format(m=m)
             } for m in self.messages.all()]
         }