Revert "emacs-lisp-mode: Use error face for "/d" indicator"

This reverts commit ddc875d9f7.

I disagree with the need to make it more prominent than the
warning face.  The warning face is prominent enough.  We
never decided to annoy people like that.

lisp/progmodes/elisp-mode.el

@@ -283,7 +283,7 @@ Comments in the form will be lost."
                      (:propertize "/d"
                       help-echo "Using old dynamic scoping mode\n\
 mouse-1: Enable lexical-binding mode"
-                      face error
+		      face warning
 		      mouse-face mode-line-highlight
                       local-map ,elisp--dynlex-modeline-map)))
   "Major mode for editing Lisp code to run in Emacs.