Remove noExitRuntime from closure-externs.js. NFC (#20322)

I believe the need to this was removed back in #10158

Author: sbc100

src/closure-externs/closure-externs.js

@@ -229,14 +229,6 @@ var outerHeight;
 var event;
 var devicePixelRatio;
 
-// TODO: Use Closure's multifile support and/or migrate worker.js onmessage handler to inside the MODULARIZEd block
-// to be able to remove all the variables below:
-
-// Variables that are present in both output runtime .js file/JS lib files, and worker.js, so cannot be minified because
-// the names need to match:
-/** @suppress {duplicate} */
-var noExitRuntime;
-
 /*
  * AudioWorkletGlobalScope globals
  */