Revert "Use `xdg-open' to open files where available"

This reverts commit 15ae89b3949f52db1b5579e383c2d9f6c9173992.

lisp/org.el

@@ -1894,13 +1894,9 @@ single keystroke rather than having to type \"yes\"."
   :type 'regexp)
 
 (defconst org-file-apps-defaults-gnu
-  (append
-   '((remote . emacs))
-   (if (executable-find "xdg-open")
-       '((system . "xdg-open %s")
-	 (t . "xdg-open %s"))
-     '((system . mailcap)
-       (t . mailcap))))
+  '((remote . emacs)
+    (system . mailcap)
+    (t . mailcap))
   "Default file applications on a UNIX or GNU/Linux system.
 See `org-file-apps'.")