[KEYS] remove keys on master branch

[szha]

Signed-off-by: Sheng Zha zhasheng@amazon.com

Description

This PR removes the KEYS for signing release packages since it should only be kept on apache dist release repo.

Checklist

Essentials

• PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• Changes are complete (i.e. I finished coding on this PR)

Changes

• include all keys in other branches in repos.

[mxnet-bot]

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