ox-md: Fix error when exporting underline objects

* lisp/ox-md.el (md): Delegate underscore transcoding to HTML
  back-end.

lisp/ox-md.el

@@ -69,7 +69,6 @@ This variable can be set to either `atx' or `setext'."
 		(org-open-file (org-md-export-to-markdown nil s v)))))))
   :translate-alist '((bold . org-md-bold)
 		     (code . org-md-verbatim)
-		     (underline . org-md-verbatim)
 		     (comment . (lambda (&rest args) ""))
 		     (comment-block . (lambda (&rest args) ""))
 		     (example-block . org-md-example-block)