SPRINGFIELD — Sen. Donald F. Harmon, keeping in line with Gov. J.B. Pritzker’s campaign promise to do away with the state’s flat income tax rate, has made the first move to amend the Illinois Constitution’s tax provision.The Oak Park Democrat filed Senate Joint Resolution 1 last week — branded as the “fair-tax” amendment. It asks voters to change the state constitution to allow for a progressive tax system.“We have a very inflexible tax policy, in large part because of the …