Remove stream producer when cleaning pubsub cache

[benjaminpetit]

Fix #7793

In PersistentStreamPullingAgent:

• Removed the logic to remove from the pubsub cache when no consumers are registered: if a stream is active but has no subscribers, it will avoid multiple useless calls to the pubsub.
• Remove the pulling agent as a producer when cleaning the pubsub cache. It will fix #7793 and should avoid useless calls from the pubsub to the pulling agent, since calls to add/remove consumers from the pubsub are ignore by the pulling agent if the stream was not in the cache.

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