Remove user_classes_count from heartbeat payload

[mboutet]

user_classes_count is already part of the stats payload so having it in the heartbeat payload is redundant and makes the payload larger.

TODO:

• Add/update test to validate the content of the heartbeat payload

[cyberw]

You should probably also remove these lines from runner.py (910-912):

                    user_classes_count = msg.data.get("user_classes_count")
                    if user_classes_count:
                        c.user_classes_count = user_classes_count

[cyberw]

ok to merge? (you still have it marked as draft)

[mboutet]

Yes