Remove dead code from rfc2047

* lisp/mail/rfc2047.el (rfc2047-fold-field): Remove dead code.

It's been disabled since 2005, when I made the change with the
following comment.

(rfc2047-encode-message-header): Disabled header folding -- not
all headers can be folded, and this should be done by the message
composition mode.  Probably.  I think.

lisp/mail/rfc2047.el

@@ -281,17 +281,7 @@ Should be called narrowed to the head of the message."
 			(encode-coding-region
 			 (point-min) (point-max)
 			 (mm-charset-to-coding-system
-			  (car message-posting-charset))))
-		  ;; No encoding necessary, but folding is nice
-		  (when nil
-		    (rfc2047-fold-region
-		     (save-excursion
-		       (goto-char (point-min))
-		       (skip-chars-forward "^:")
-		       (when (looking-at ": ")
-			 (forward-char 2))
-		       (point))
-		     (point-max))))
+			  (car message-posting-charset)))))
 	      ;; We found something that may perhaps be encoded.
 	      (re-search-forward "^[^:]+: *" nil t)
 	      (cond