org-manual: Delete reference to removed function

* doc/org-manual.org (Inserting deadlines or schedules): Remove
  reference to `org-mark-entry-for-agenda-action`.

Reported-by: Christian Heinrich <com-orgmode@gladbachcity.de>
<http://lists.gnu.org/r/emacs-orgmode/2019-05/msg00060.html>

doc/org-manual.org

@@ -6153,17 +6153,6 @@ schedule an item:[fn:67]
      variable ~org-log-reschedule~, take a note when changing an
      existing scheduling time[fn:69].
 
-- {{{kbd(C-c C-x C-k)}}} (~org-mark-entry-for-agenda-action~) ::
-
-     #+kindex: C-c C-x C-k
-     #+kindex: k a
-     #+kindex: k s
-     #+findex: org-mark-entry-for-agenda-action
-     Mark the current entry for agenda action.  After you have marked
-     the entry like this, you can open the agenda or the calendar to
-     find an appropriate date.  With point on the selected date, press
-     {{{kbd(k s)}}} or {{{kbd(k d)}}} to schedule the marked item.
-
 - {{{kbd(C-c / d)}}} (~org-check-deadlines~) ::
 
      #+kindex: C-c / d