You will not be able to remove the Remote Management as only the company that manages the device is able to remove it. Removing the SIM card will not solve the problem. Was this a company supplied device or your personal device purchased from Apple or Authorized Apple seller? I would not allow Remote software to be installed on my personal device and it should only be done on a device that is supplied by the company. You can contact Apple directly for further assistance and be prepared to provide the original purchase receipt from Apple for the device, but have not seen where they are able to remove the RM either. They will also be able to advise on how it affects trade-in value. I would physically take the device to the store for trade-in instead of the mail in option. By taking it to the store, you will get an instant trade in value, where mailing it in may be subject to reduced value after they receive the device.
Contact Apple for support and service - Apple Support