(next-history-element): Use minibuffer-prompt-end once

again.  Remove test for minibuffer-prompt-in-buffer.

lisp/simple.el

@@ -794,9 +794,7 @@ An uppercase letter in REGEXP makes the search case-sensitive."
 	     (let ((print-level nil))
 	       (prin1-to-string elt))
 	   elt))
-	(if (boundp 'minibuffer-prompt-in-buffer)
-	    (goto-char (minibuffer-prompt-width))
-	  (goto-char (point-min))))))
+	(goto-char (minibuffer-prompt-end)))))
 
 (defun previous-history-element (n)
   "Inserts the previous element of the minibuffer history into the minibuffer."