Hi -
You would need to add an unbound Post Process volume into your level and zero out all of the numbers. You will need to make sure that you click the check box or default settings will be used. Also note that the Tonemapper is required by the engine and cannot be removed. You can however go into the Project Settings and uncheck Mobile HDR which will only render lighting using LDR (effectively “removing” the tonemapper).
Thank You
Eric Ketchum