Ownership Rule Unassign attribute

### Problem Statement I'm refactoring issue ownership and would like the ability to unassign issues with an existing owner. To achieve my goal I have 2 options: - Updating an issue through Sentry's APIs - Manually unassigning each issue through the UI ### Solution Brainstorm Leveraging ownership rules potentially I could assign a rule to `none` or empty( " ") owner. ### Product Area Issues