Removed no longer used externs.js

utils/build/externs.js

@@ -1,10 +0,0 @@
-var THREE;
-var define;
-var module;
-var exports;
-var performance;
-var createImageBitmap;
-var WebGL2RenderingContext;
-var WebGL2ComputeRenderingContext;
-var XRHand;
-var XRWebGLLayer;