Remove 'msg' parameter from convert_tuples_by_name
The message was included as a parameter when this function was added in
dcb2bda9b704, but I don't think it has ever served any useful purpose.
Let's stop spreading it pointlessly.
Reviewed by Amit Langote and Peter Eisentraut.
Discussion: https://postgr.es/m/
20190806224728.GA17233@alvherre.pgsql