Remove PDFViewerApplication.initPassiveLoading and directly invoke the open-method from the extension-specific code

This old method is essentially just adding, a small amount of, unnecessary indirection and we can easily invoke `PDFViewerApplication.open` directly from the extension-specific code instead.

web/external_services.js

@@ -24,7 +24,7 @@ class BaseExternalServices {
 
   updateFindMatchesCount(data) {}
 
-  initPassiveLoading(callbacks) {}
+  initPassiveLoading() {}
 
   reportTelemetry(data) {}