chore: add unregister method

[marandaneto]

:bulb: Motivation and Context

Add unregister method.

Follow up https://posthog.slack.com/archives/C04MZFDA5KK/p1709027632605249 the user didn't need to reset, but could register and unregister something.

:green_heart: How did you test it?

:pencil: Checklist

• [X] I reviewed the submitted code.
• [ ] I added tests to verify the changes.
• [ ] I updated the docs if needed.
• [X] No breaking change or entry added to the changelog.

[marandaneto]

Your changes look good, so happy if you want to merge this or hold off until the strange behaviour is understood better

will merge it, if there's something off is on the JS SDK, and it works well for Android/iOS (main targets anyway).