org-table.el: Fix orgtbl-mode not remapping delete key

* lisp/org-table.el (orgtbl-setup): Remap `delete-forward-char' in
addition to `delete-char'.  The former is called in Emacs >26.

Partially fixes
https://orgmode.org/list/1AD1B9B7-5A15-4F86-9274-B04B83694C85@misasa.okayama-u.ac.jp

lisp/org-table.el

@@ -5290,6 +5290,7 @@ to execute outside of tables."
       (org-remap orgtbl-mode-map
 		 'self-insert-command 'orgtbl-self-insert-command
 		 'delete-char 'org-delete-char
+                 'delete-forward-char 'org-delete-char
 		 'delete-backward-char 'org-delete-backward-char)
       (org-defkey orgtbl-mode-map "|" 'org-force-self-insert))
     t))