[Idea] Remove sOldPreference and compatibility with original Ri-Li versions

[zabidenhtf]

Now original Ri-Li project is dead, and compatibility is nonsense.

[petitlapin]

It's a bit more complicated than this :). The "new" format was introduced in f7ce97c and is available since version v3.1.4. So if anyone has a previous version of Li-Ri, they have the "oldPreferences" and updating Li-Ri to a newer version, they would expect to keep their preferences (highscore, max level...).

I agree it should be cleaned at some point, but it may be too early. One solution to not forget to clean would be to create the PR with a target release, this way we can merge it when we are ready (I would say we can target the end of the year)