docs: remove some system Python notes

[cho-m]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

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Last system Python was Monterey 12.2. For last 3 supported macOS, it is possible to still have system Python if user is on Monterey and never upgraded to 12.3+, but I think we can phase out these comments.

[MikeMcQuaid]

Thanks again @cho-m!