Make unlit materials, and do not place any light in level.
But that unlit way is very extreme cutting off everything that is light related.
For normal ways of optimizing use static lights only and set details while creating project set it to mobile and scalable quality. Then you should look for rendering settings for project.
Ps.
You can set unlit mode for viewport in editor, and there should be also some console command for unreal projects that forces them to unlit rendering.