Make it possible to unset a featured image via the block

[jameskoster]

I think we should revert #27291.

It seems perfectly reasonable to offer folks a way to unset the featured image via the block itself. Otherwise one has to go into the post settings of the Inspector to do so. That might involve quite a journey if you're editing something like a query in the Index template!

It would be much more convenient to do this via the block.

[Mamaduka]

I can create PR for this shortly if no one else is working on it.