[BEAM-4790] Remove CODEOWNERs.

[udim]

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Follow this checklist to help us incorporate your contribution quickly and easily:

• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

It will help us expedite review of your Pull Request if you tag someone (e.g. @username) to look at it.

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---	---	---	---	---
Java
Python		---		---	---	---	---

[udim]

R: @charlesccychen