bug 1336901 - remove incorrect assert r=yzen

There are cases where there is no good reason to shutdown the child document
actors before the parent.  For example when TabParent::DestroyInternal() is
about to send the Destroy message we might as well just shut down the top level
document and let that tear down the rest.

accessible/ipc/DocAccessibleParent.cpp

@@ -461,8 +461,6 @@ DocAccessibleParent::Destroy()
     return;
   }
 
-  NS_ASSERTION(mChildDocs.IsEmpty(),
-               "why weren't the child docs destroyed already?");
   mShutdown = true;
 
   uint32_t childDocCount = mChildDocs.Length();