Remove temporary KafkaSourceDescriptor.create function

[HurSungYun]

In b609aab (#12518), it is added to make a workaround for bug in AutoValueSchema. It seems fixed now (#12520), so I suggest to remove this function.

Feel free to close this PR if I got something wrong.

I found this code change by investigating my issue, which is #36356. When updating Dataflow job, it occurred NullPointerException because of topic is null. (Drain and re-deploy worked well, but it happens only in existing job update scenarioes)

I doubt this code change would resolve the original issue, but I believe this change might be needed in any way.

Related PRs -> #36295

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[sjvanrossum]

Note for reviewers: I can't recall if these objects get serialized into the pipeline graph, but if that's the case then we can't remove this static method because the auto-generated serialVersionUID would change (see https://docs.oracle.com/javase/8/docs/platform/serialization/spec/class.html#a4100) and break update compatibility. 🙃

[kennknowles]

I think they may be both serialized to the graph and shipped in Kafka SDF using SerializableCoder (haven't checked). So, yes, this could be an issue.

[HurSungYun]

@kennknowles So, could you please tell me what should I do now?

If this change breaks something, it would be better to just comment not to change this due to backward-compatibility, and just keep use KafkaSourceDescriptor.create function.

This PR might mitigate potential issues in any way.

Please feel free to close this PR if needed.

[kennknowles]

Yea it is pretty hard to test. You would have to start a pipeline with the released version of the SDK and then use this version of the SDK to update. Right now we don't have good automation around it in the Beam project.

It is OK to break update compatibility but we have to put in a flag like here: https://github.com/apache/beam/pull/28853/files#diff-b8cf6c3051a36c566f2f28f525449f456a88b05b3b4c17c814e6a55ba2ce36e9R77-R90

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[damccorm]

R: @kennknowles to silence the bot

I generally agree with the comments above though - I don't think this is update-safe in its current form; we should either do nothing (which is probably fine here) or follow Kenn's comments here - #36357 (comment) (this may add more complexity than it is worth though)

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[HurSungYun]

I agree with all the comments above, so please feel free to close this PR if needed.