How can I remove the small map window in the app interface of the DJI GO ?

Touch the top left corner and it shrinks to a small icon.

You cannot remove the map screen completely when using the DJI Go app, you can only minimize it to a small corner screen. If you have a Phantom unit that has a HDMI output module installed, you can split to a HDMI monitor that would not have a map available and only the camera view.