Bug 1834874 - P2: Remove DocAccessiblePlatformExt. r=Jamie,ipc-reviewers,mccr8

Since we removed the last dependency on this subprotocol, we can now
remove it.

Differential Revision: https://phabricator.services.mozilla.com/D181321

accessible/base/NotificationController.cpp

@@ -991,9 +991,6 @@ void NotificationController::WillRefresh(mozilla::TimeStamp aTime) {
             ->SendPDocAccessibleConstructor(
                 ipcDoc, parentIPCDoc, id,
                 childDoc->DocumentNode()->GetBrowsingContext());
-#ifndef XP_WIN
-        ipcDoc->SendPDocAccessiblePlatformExtConstructor();
-#endif
       }
     }
   }