[ie/triller] Fix unlisted video extraction

[bashonly]

Triller's brilliant API started returning a string (not boolean) value of "true"/"false" in the followed_by_me field, which made the error handling for private profiles useless.

This PR also removes the private check from video extraction, since apparently "private" actually means "unlisted", and private videos can be watched/downloaded as long as the video URL is known.

Template

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• Fix or improvement to an extractor (Make sure to add/update tests)
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• Core bug fix/improvement
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Copilot Summary

🤖 Generated by Copilot at 3d55bfe

Summary

🔀🛡️🧰

Refactored the Triller extractor to handle privacy and block checks in the specific subclasses. Simplified the user info extraction with traverse_obj.

Sing, O Muse, of the skillful coder who refined
The Triller extractor, and from the base class removed
The _check_user_info method, that no longer suited
The subclasses, where the privacy and block status he proved.

Walkthrough

• Remove _check_user_info method from TrillerBaseIE class, as it is no longer used by subclasses (link)
• Simplify logic for checking user info in _real_extract method of TrillerIE class, using API response fields directly (link)
• Improve error handling and field parsing in _real_extract method of TrillerUserIE class, using traverse_obj utility function and converting string values to booleans (link)