Remove useless binding

* lisp/org.el (org-mode-map): Remove [(meta return)] binding, handled
  by (kbd "M-RET").

Suggested-by: Allen Li <vianchielfaura@gmail.com>
<http://lists.gnu.org/archive/html/emacs-orgmode/2017-09/msg00411.html>

lisp/org.el

@@ -19481,7 +19481,6 @@ COMMANDS is a list of alternating OLDDEF NEWDEF command names."
 
 (org-defkey org-mode-map [(shift return)]   'org-table-copy-down)
 (org-defkey org-mode-map [(meta shift return)] 'org-insert-todo-heading)
-(org-defkey org-mode-map [(meta return)]       'org-meta-return)
 (org-defkey org-mode-map (kbd "M-RET") #'org-meta-return)
 
 ;; Cursor keys with modifiers