[api-minor] Remove the structTreeLayerFactory in the viewer

Please note that this functionality has never really mattered for the Firefox PDF Viewer, the GENERIC viewer, or even the "simpleviewer"/"singlepageviewer" component-examples. Hence, in practice this means that only the "pageviewer" component-example[1] have ever really utilized this.

Using factories to initialize various layers in the viewer, rather than simply invoking the relevant code directly, seems (at least to me) like a somewhat roundabout way of doing things.
Not only does this lead to more code, both to write and maintain, but since many of the layers have common parameters (e.g. an `AnnotationStorage`-instance) there's also some duplication.

Hence this patch, which removes the `structTreeLayerFactory` and instead uses a lookup-function in the `PDFPageView`-class to access the external viewer-properties as necessary.
Note that this should even be an improvement for the "pageviewer" component-example, since most layers will now work by default rather than require manual configuration.

---
[1] In practice we generally suggest using the "simpleviewer", or "singlepageviewer", since it does *most* things out-of-the-box and given that a lot of functionality really require *a viewer* and not just a single page in order to work.

web/pdf_viewer.component.js

@@ -14,7 +14,6 @@
  */
 
 import {
-  DefaultStructTreeLayerFactory,
   DefaultTextLayerFactory,
   DefaultXfaLayerFactory,
 } from "./default_factory.js";
@@ -60,6 +59,15 @@ class DefaultAnnotationLayerFactory {
   }
 }
 
+class DefaultStructTreeLayerFactory {
+  constructor() {
+    throw new Error(
+      "The `DefaultStructTreeLayerFactory` has been removed, " +
+        "this functionality is automatically enabled when the TextLayer is used."
+    );
+  }
+}
+
 export {
   AnnotationLayerBuilder,
   DefaultAnnotationLayerFactory,