Move to drop support for nlohmann::json in favor of variant
If the final aim is to have YARR only rely on using variant, and if variant is going to be added to TDAQ release and then used in operation, we might as well remove the bandaid already and drop support for
nlohmann::json. Right now, the user can choose at compile time whether or not to use
variant. This option should just be removed.
The only problem is that
variant has absolutely no documentation/API reference, which if it is to be used in production is not ideal. If we move to using it early, perhaps we (as YARR users) can start adding some work on this front?