Remove the unused `isWorker` property from `src/display/global.js`

This has been completely unused since commit 1d12aed5ca, in PR 7126, almost one and a half years ago.

src/display/global.js

@@ -34,8 +34,6 @@ import { Metadata } from './metadata';
 import { renderTextLayer } from './text_layer';
 import { SVGGraphics } from './svg';
 
-var isWorker = (typeof window === 'undefined');
-
 // The global PDFJS object is now deprecated and will not be supported in
 // the future. The members below are maintained for backward  compatibility
 // and shall not be extended or modified. If the global.js is included as
@@ -306,6 +304,5 @@ PDFJS.UnsupportedManager = _UnsupportedManager;
 
 export {
   globalScope,
-  isWorker,
   PDFJS,
 };