Remove old issues workflow (radius-project#7020)

# Description

This PR removes a stale workflow that we used to leverage prior to
launching. It references teams and labels that no longer exist and is no
longer needed.

## Type of change

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- This pull request is a minor refactor, code cleanup, test improvement,
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Signed-off-by: Aaron Crawfis <[email protected]>
Co-authored-by: Young Bu Park <[email protected]>