FrontFacingNode: Fix `frontFacing` if used `material.flatShading` (#30828)

src/nodes/display/FrontFacingNode.js

@@ -38,8 +38,6 @@ class FrontFacingNode extends Node {
 
 		const { renderer, material } = builder;
 
-		if ( material.flatShading === true ) return 'true';
-
 		if ( renderer.coordinateSystem === WebGLCoordinateSystem ) {
 
 			if ( material.side === BackSide ) {