Remove support for legacy visible key

[liammulh]

There are sim string keys that have a legacy visible key. For example, see:

https://github.com/phetsims/forces-and-motion-basics/blob/cf9242a8863dfecd8eec669b5dd3bcca4bb77b5f/forces-and-motion-basics-strings_en.json#L75-L78

For more context on this issue, see phetsims/forces-and-motion-basics#298.

Once we are sure all of these legacy visible keys have been removed, we can remove the support for them in Rosetta, which will slightly reduce the complexity of our code.