Always try to display tree-widget with images (Bug#36147)

Always try to display tree-widget with images (Bug#36147)

Checking the result of display-images-p doesn't make sense if the
display capabilities when loading the file are different from the
display used to actually look at the widget (which is easily possible
now that Emacs supports both tty and graphic frames in the same
instance).
* lisp/tree-widget.el (tree-widget-image-enable): Default to t,
always.  This still shows text on displays that can't show images.