fix @makc's issue #6109 with "attribute vec2 uv" being defined even when not needed.

src/renderers/shaders/ShaderChunk/uv2_pars_vertex.glsl

@@ -1,5 +1,6 @@
 #if defined( USE_LIGHTMAP ) || defined( USE_AOMAP )
 
+	attribute vec2 uv2;
 	varying vec2 vUv2;
 
 #endif

src/renderers/webgl/WebGLProgram.js

@@ -221,7 +221,6 @@ THREE.WebGLProgram = ( function () {
 				'attribute vec3 position;',
 				'attribute vec3 normal;',
 				'attribute vec2 uv;',
-				'attribute vec2 uv2;',
 
 				'#ifdef USE_COLOR',