ci: remove maintainer PR Slack notification workflow

[reinkrul]

Summary

• Removes the "Notify on maintainer PR" workflow, which posted a Slack notification when a maintainer opened/readied a PR.
• Not used by maintainers to actually review PRs, so it's just noise.

Test plan

• N/A (workflow removal only)

[qltysh]

Coverage Impact

This PR will not change total coverage.

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  • Indirect Changes: Changes to File Coverage for files that were not modified in this PR. Learn more.