Remove the eventBus parameter from getViewerConfiguration

This structure contains *almost* exclusively references to DOM elements (and a couple of simple strings), rather than complete classes/functions. Hence the `eventBus`-option sticks out a fair bit, and I'd guess that it's *mostly* unused in e.g. third-party implementations.

Given that we, in multiple places, mention that the default viewer shouldn't be used as-is I really don't think that we need to keep this special `eventBus`-option around. Furthermore, nowadays it's also a lot easier to (safely) access the existing `EventBus`-instance in the viewer; see https://github.com/mozilla/pdf.js/wiki/Third-party-viewer-usage#initialization-promise which shows how to listen for the default viewer being initialized (and its `eventBus` thus being available).

web/app.js

@@ -463,14 +463,9 @@ const PDFViewerApplication = {
   async _initializeViewerComponents() {
     const { appConfig, externalServices } = this;
 
-    let eventBus;
+    const eventBus = externalServices.isInAutomation
-    if (appConfig.eventBus) {
+      ? new AutomationEventBus()
-      eventBus = appConfig.eventBus;
+      : new EventBus();
-    } else if (externalServices.isInAutomation) {
-      eventBus = new AutomationEventBus();
-    } else {
-      eventBus = new EventBus();
-    }
     this.eventBus = eventBus;
 
     this.overlayManager = new OverlayManager();

web/viewer.js

@@ -77,7 +77,6 @@ function getViewerConfiguration() {
     appContainer: document.body,
     mainContainer: document.getElementById("viewerContainer"),
     viewerContainer: document.getElementById("viewer"),
-    eventBus: null,
     toolbar: {
       container: document.getElementById("toolbarViewer"),
       numPages: document.getElementById("numPages"),