[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/interfaces.js

@@ -17,8 +17,6 @@
 /** @typedef {import("../src/display/api").PDFPageProxy} PDFPageProxy */
 // eslint-disable-next-line max-len
 /** @typedef {import("../src/display/display_utils").PageViewport} PageViewport */
-// eslint-disable-next-line max-len
-/** @typedef {import("./struct_tree_builder").StructTreeLayerBuilder} StructTreeLayerBuilder */
 /** @typedef {import("./text_highlighter").TextHighlighter} TextHighlighter */
 // eslint-disable-next-line max-len
 /** @typedef {import("./text_layer_builder").TextLayerBuilder} TextLayerBuilder */
@@ -198,16 +196,6 @@ class IPDFXfaLayerFactory {
   createXfaLayerBuilder({ pageDiv, pdfPage, annotationStorage = null }) {}
 }
 
-/**
- * @interface
- */
-class IPDFStructTreeLayerFactory {
-  /**
-   * @returns {StructTreeLayerBuilder}
-   */
-  createStructTreeLayerBuilder() {}
-}
-
 /**
  * @interface
  */
@@ -278,7 +266,6 @@ export {
   IDownloadManager,
   IL10n,
   IPDFLinkService,
-  IPDFStructTreeLayerFactory,
   IPDFTextLayerFactory,
   IPDFXfaLayerFactory,
   IRenderableView,