Final cleanup of JS code

[clangenb]

Almost everything is done now. There are just a some minor js-calls that I have overlooked so far:

    final info = await Future.wait([
      evalJavascript('settings.getNetworkConst()'),
      evalJavascript('api.rpc.system.properties()'),
      evalJavascript('api.rpc.system.chain()'), // "Development" or "Encointer Testnet Gesell" or whatever
    ]);

I will check if we even need them, and either port them to dart, or remove them if we don't need the values.

Afterward, we can:

• Delete all the JS code
• Remove all the dependencies required to run the JS code.