Remove my own implementation of duplicate-line

Emacs has it built in now.

configuration.org

@@ -369,23 +369,6 @@ We need to do things differently, if so.
   (kill-line 1))
 #+END_SRC
 
-*** Duplicate current line
-
-#+BEGIN_SRC emacs-lisp
-(defun gpolonkai/duplicate-line()
-  "Duplicate line at point."
-
-  (interactive)
-
-  (save-excursion
-    (move-beginning-of-line 1)
-    (kill-line)
-    (yank)
-    (open-line 1)
-    (forward-line 1)
-    (yank)))
-#+END_SRC
-
 *** Toggle case of character at point
 
 Based on [[http://ergoemacs.org/emacs/modernization_upcase-word.html][Xah’s toggle letter case defun version 2015-12-22]]
@@ -3958,7 +3941,7 @@ directory.  It is available from [[http://plantuml.com/download][here]].
  ("M-g SPC" . goto-next-char)
  ("M-F" . gpolonkai/beginning-of-next-word)
  :map ctl-x-map
- ("C-y" . gpolonkai/duplicate-line)
+ ("C-y" . duplicate-line)
  ("_" . maximize-window)
  ("C-r" . rename-current-buffer-file)
  ("C-d" . delete-current-buffer-file)