Most likely you were “grandfathered” in for the endorsement. To remove it you would need to complete the “towing” exam module. It’s not too clear but NMC will also probably require a TOAR, unless you already completed one. Another way to remove it is to upgrade your Master to 500 or 1600, and that should remove the tonnage limitation on the Master of Towing. There is a slight disconnect in the regs for this situation as many mariners were grandfathered in during the phase in period of the Towing regs. Now it’s pretty clear how you have to obtain the endorsement, but the NVIC had a flow chart to show how to grandfather someone in, but it didn’t clarify how to remove the tonnage limitation other than the exams.
Does your Domestic Master of Towing have a tonnage limitation?