Deprecate QStyle::State_Editing

I think QStyle::State_Editing should be deprecated.

1. All the code related to QStyle::State_Editing is dead.
2. It's been broken since at least Qt 4.5.
3. The same result can be achieved with isPersistentEditorOpen since Qt 5.10.
4. Making it work correctly will add overhead with no tangible benefit.
5. I was confused by the documentation. I know other people who've also been confused by it.

It seems like the way editing is handled was changed a long time ago but the rest of the code was never updated.

This was motivated by trying to fix it in QTBUG-68947 but realizing that wasn't the best solution.