Update WebGLRenderer.js

Optimizing compiler best practice. Don't change the shape of objects.

src/renderers/WebGLRenderer.js

@@ -3210,7 +3210,7 @@ THREE.WebGLRenderer = function ( parameters ) {
 
 			if ( dispose && ! attributeItem.dynamic ) {
 
-				delete attributeItem.array;
+				attributeItem.array = null;
 
 			}