embed: Remove useless 'sender' field.

The variant `update_message` events have this extra sender field not
present in normal update_message events; this field has no purpose, so
we remove it.

zerver/lib/actions.py

@@ -4403,7 +4403,6 @@ def do_update_embedded_data(user_profile: UserProfile,
                             rendered_content: Optional[str]) -> None:
     event: Dict[str, Any] = {
         'type': 'update_message',
-        'sender': user_profile.email,
         'message_id': message.id}
     changed_messages = [message]
 

zerver/lib/event_schema.py

@@ -761,7 +761,6 @@ check_update_message_embedded = check_events_dict(
         ("message_id", check_int),
         ("message_ids", check_list(check_int)),
         ("rendered_content", check_string),
-        ("sender", check_string),
     ]
 )