Revert "lightplay extension stub"

This reverts commit 36de7517bd.

src/extension-support/extension-manager.js

@@ -25,7 +25,6 @@ const builtinExtensions = {
     boost: () => require('../extensions/scratch3_boost'),
     gdxfor: () => require('../extensions/scratch3_gdx_for'),
     synth: () => require('../extensions/scratch3_synth'),
-    lightplay: () => require('../extensions/scratch3_lightplay'),
 };
 
 /**