Even by default there will always be a GameMode set, even if it is the default “GameMode”. You shouldn’t need to set it to none, but if you do want all of the defaults to be none, you can set those in the Project Settings. Open your project and then select (from the top left) Edit > Project Settings > Maps & Modes > Default GameMode. From here, you can select the GameMode you want. After that, select the down arrow beside Selected GameMode and you can set the individual settings for this GameMode. You can set all of these to none if you wish.