Remove the warning of missing model mapping

[phackstock]

This causes more harm than good, should be removed.

[danielhuppmann]

I believe that you refer to this warning:

No model mapping found for model ...

I'd still keep it but rephrase to

Skipping region aggregation for model '{}' (no region processing mapping)

to make it sound less like an error...

[phackstock]

@danielhuppmann, yes that was the warning I was referring to. Might be a good first, albeit super short, nomenclature contribution for @dc-almeida.