display: fix assert when window fails to open

The assert is being triggered because the window is already being removed in open_windows.  It does not really matter if it returns false anyway, as long as the window is removed one way or another.

panda/src/display/graphicsEngine.cxx

@@ -445,8 +445,7 @@ make_output(GraphicsPipe *pipe,
       }
 
       // No good; delete the window and keep trying.
-      bool removed = remove_window(window);
-      nassertr(removed, NULL);
+      remove_window(window);
     }
   }