In the DSD+ GUI, navigate to MISC > Tag TDMA P25 TGs. Make sure that it is unchecked, and it will stop appending it to new TDMA groups.
Additionally, if you want to remove the "(TDMA)" appended to each existing group, you will need to shut down DSD+ and then edit the .groups file to remove it. I use Notepad++. Save the groups file, then run DSD+. It should work.