While the New York Knicks were battling to win the NBA Finals earlier this month, Mitch Modell landed in the middle of a testy exchange with the new owner of Modell’s Sporting Goods — capping months of failed negotiations to buy back the faded retail brand, The Post has learned.  The former “Undercover Boss” and CEO of the sporting-goods chain, which went bankrupt in 2020 and closed 140 stores , on Monday revealed MitchellModells.com , a new online apparel site that’s selling a T-shirt