3.x: unnecessary call to upstream .cancel() in FlowablePublishMulticast operator

[sobersanta]

I think I should add it as a separate issue.

In a casual completion chain from the upstream, when it is calling onComplete/onError, FlowablePublishMulticast calls SubscriptionHelper.cancel(upstream); in the dispose() method (line 196).
I believe that upstream cancellation should be done only from .cancel() method not from .onComplete or .onError as it causes issues in some scenarios including extra groups creation in the groupBy operator.