"Mosaico" model should not ask for API Base + Reference Client URL

When creating a new "Mosaico" model, it is still asking for the "API Base" and "Reference Client URL" options. These options are actually the same, and they are also redundant, since the reference client is the one starting the reference agent itself.

These options should be removed, and the reference client would be responsible for computing the correct URL to use to communicate with the reference agent that it starts itself.