SDK method for unset person props

### Description

context https://github.com/PostHog/posthog-android/pull/369#discussion_r2698309797

- Android (https://github.com/PostHog/posthog-android/issues/370)
- iOS (this issue)
- RN (https://github.com/PostHog/posthog-js/issues/3812)
- Flutter (https://github.com/PostHog/posthog-flutter/issues/426)
- JS (https://github.com/PostHog/posthog-js/issues/3720)
- [ ] Prob all other SDKs