org-agenda: Fix consecutive bulk marks on the same task

* lisp/org-agenda.el (org-agenda-bulk-unmark-all): Do not reset
  markers stored in `org-agenda-bulk-marked-entries'.  These markers
  are not specific to bulk actions and need not be modified by
  side-effect.

Reported-by: Adrian Bradd <adrian.bradd@gmail.com>
<http://permalink.gmane.org/gmane.emacs.orgmode/112895>

lisp/org-agenda.el

@@ -9869,7 +9869,6 @@ This will remove the markers and the overlays."
   (interactive)
   (if (null org-agenda-bulk-marked-entries)
       (message "No entry to unmark")
-    (mapc (lambda (m) (move-marker m nil)) org-agenda-bulk-marked-entries)
     (setq org-agenda-bulk-marked-entries nil)
     (org-agenda-bulk-remove-overlays (point-min) (point-max))))