fix(firebase_database): Fix MissingPluginException caused by malformed EventChannel name.

[russellwheatley]

Description

In retrospect, I don't think we need to track the event channel names at all. In other plugins, we keep a map of stream handlers so we can clean up. In this implementation, we pass a "dispose of" function to the event channel to clean up the stream handler. So the event names aren't used for anything.

So, I think we can remove the queryListeners and associated code. Thoughts, @Salakar?

Will roll out for iOS once I have a second opinion 😄.

Update

Every event stream has a new number id for android & iOS.

Related Issues

fixes #7795
fixes #7556

I suspect it fixes this issue as well: #7618

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