A judge in the Georgia election interference case against former President Donald J. Trump heard final arguments on Friday on a motion to disqualify the prosecutor who brought the case, Fani T. Willis, on the ground that a romantic relationship she had with a subordinate created a conflict of interest.With a historic criminal case against a former president on the line, lawyers for Mr. Trump and his co-defendants took turns assailing Ms. Willis. John B. Merchant III, who represents the defendant Michael Roman, said that if the court finds the prosecutors’ romance did not create a…