MeshPhysicalMaterial: Reverted to @WestLangley specularColor formula.

src/renderers/shaders/ShaderChunk/lights_physical_fragment.glsl

@@ -1,4 +1,4 @@
 PhysicalMaterial material;
 material.diffuseColor = diffuseColor.rgb * ( 1.0 - metalnessFactor );
 material.specularRoughness = roughnessFactor * 0.5 + 0.5; // disney's remapping of [ 0, 1 ] roughness to [ 0.5, 1 ]
-material.specularColor = mix( vec3( 0.04 ) * reflectivity, diffuseColor.rgb, metalnessFactor );
+material.specularColor = mix( vec3( 0.04 ), diffuseColor.rgb, metalnessFactor );