I was making a Alt-History Mod then when I play the game, there is a DMZ in the rhineland. But I also created a new Germany Focus Tree so How could I remove it?

You would need to edit the state's file in your mod.

If you go to "YOUR_MODS_FOLDER/history/states" and go to state number 51, inside there is a line "set_demilitarized_zone..." If you remove it, then it will remove the DMZ.

If your mod doesn't have that folder, create it, and copy the file from the game's folder to your mod, and then edit it.

Thanks!