Remove the IMAGE_DECODERS special-case when polyfilling `structuredCl…

…one`

Originally we only used the `structuredClone` polyfill in the `LoopbackPort`-implementation, and that obviously isn't used anywhere within the various image decoders.
At this point in time we've started to use `structuredClone` a little bit more, hence it seems overall simpler to just bundle the polyfill even in the `legacy`-version of the IMAGE_DECODERS built-target.

src/shared/compatibility.js

@@ -54,11 +54,6 @@ import { isNodeJS } from "./is_node.js";
 
 // Support: Chrome<98, Safari<15.4
 (function checkStructuredClone() {
-  if (typeof PDFJSDev !== "undefined" && PDFJSDev.test("IMAGE_DECODERS")) {
-    // The current image decoders are synchronous, hence `structuredClone`
-    // shouldn't need to be polyfilled for the IMAGE_DECODERS build target.
-    return;
-  }
   if (globalThis.structuredClone) {
     return;
   }