Clean-up getMainThreadWorkerMessageHandler for non-PRODUCTION mode

*This is a final piece of clean-up of code that I recently wrote, after which I'm done :-)*

When the `getMainThreadWorkerMessageHandler` function was added, in PR 9385, it did so by basically introducing a `web/app.js` dependency in `src/display/api.js` through the `window.pdfjsNonProductionPdfWorker` property[1]. Even though this is limited to non-`PRODUCTION` mode, i.e. `gulp server`, it still seems unfortunate to have that sort of viewer dependency in the API code itself.

With the new, much nicer and shorter, names introduced in PR 9565 we can remove this non-`PRODUCTION` hack and just use `window.pdfjsWorker` in both the viewer and the API regardless of the build mode.

---

[1] It didn't seem correct to piggy-back on the `window.pdfjsDistBuildPdfWorker` property in non-`PRODUCTION` mode.

src/display/api.js

@@ -1317,13 +1317,7 @@ var PDFWorker = (function PDFWorkerClosure() {
     if (typeof window === 'undefined') {
       return null;
     }
-    if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('PRODUCTION')) {
-      return (window.pdfjsNonProductionPdfWorker &&
-              window.pdfjsNonProductionPdfWorker.WorkerMessageHandler);
-    }
-    // PRODUCTION
-    return (window.pdfjsWorker &&
-            window.pdfjsWorker.WorkerMessageHandler);
+    return (window.pdfjsWorker && window.pdfjsWorker.WorkerMessageHandler);
   }
 
   let fakeWorkerFilesLoadedCapability;

web/app.js

@@ -1524,11 +1524,11 @@ function loadFakeWorker() {
     if (typeof PDFJSDev === 'undefined' || !PDFJSDev.test('PRODUCTION')) {
       if (typeof SystemJS === 'object') {
         SystemJS.import('pdfjs/core/worker').then((worker) => {
-          window.pdfjsNonProductionPdfWorker = worker;
+          window.pdfjsWorker = worker;
           resolve();
         });
       } else if (typeof require === 'function') {
-        window.pdfjsNonProductionPdfWorker = require('../src/core/worker.js');
+        window.pdfjsWorker = require('../src/core/worker.js');
         resolve();
       } else {
         reject(new Error(