push_notification: Remove 'zulip_message_id' from FCM payload.

This commit removes the 'zulip_message_id' field from the FCM
payload for remove push notification.

Clients that use `zulip_message_ids` instead have been out for
years and we no longer support older client versions that don't.

api_docs/changelog.md

@@ -24,6 +24,10 @@ format used by the Zulip server that they are interacting with.
 
 * Mobile push notification payloads for APNs no longer contain the
   `server` and `realm_id` fields, which were unused.
+* Mobile push notification payloads for FCM to remove push
+  notifications no longer contain the legacy pre-2019
+  `zulip_message_id` field; all functional clients support the newer
+  `zulip_message_ids`.
 
 **Feature level 412**
 

zerver/lib/push_notifications.py

@@ -1203,9 +1203,6 @@ def get_remove_payload_gcm(
     gcm_payload.update(
         event="remove",
         zulip_message_ids=",".join(str(id) for id in message_ids),
-        # Older clients (all clients older than 2019-02-13) look only at
-        # `zulip_message_id` and ignore `zulip_message_ids`.  Do our best.
-        zulip_message_id=message_ids[0],
     )
     gcm_options = {"priority": "normal"}
     return gcm_payload, gcm_options

zerver/tests/test_handle_push_notification.py

@@ -655,7 +655,6 @@ class HandlePushNotificationTest(PushNotificationTestCase):
                     "user_id": self.user_profile.id,
                     "event": "remove",
                     "zulip_message_ids": str(message.id),
-                    "zulip_message_id": message.id,
                 },
                 {"priority": "normal"},
                 list(
@@ -724,7 +723,6 @@ class HandlePushNotificationTest(PushNotificationTestCase):
                     "user_id": self.user_profile.id,
                     "event": "remove",
                     "zulip_message_ids": str(message.id),
-                    "zulip_message_id": message.id,
                 },
                 {"priority": "normal"},
             )