Why need handle delete topic in topic change event

[方晓兵]

Hi Team, 

I have a problem when I study kafka code in version 2.8.0.

I see `controllerContext.removeTopic(topic)` have been called in 
`TopicDeletionManager.completeDeleteTopic()`. TopicChangeHandler not only 
listen to child delete but also listen to child add. So every time a topic is 
deleted, an invalid event will be triggered because 
`controllerContext.removeTopic(topic)` have been called in 
`TopicDeletionManager.completeDeleteTopic()` after delete this topic zookeeper 
path. Is it code that needs to be optimized?

Grateful and look forward to answers