Erase `org-insert-labeled-timestamps-at-point'
* lisp/org.el (org-insert-labeled-timestamps-at-point): Remove
variable.
(org-add-planning-info): Remove reference to removed variable.
* lisp/org-agenda.el (org-agenda-schedule, org-agenda-deadline):
Remove reference to removed variable.
* etc/ORG-NEWS (Skipping): Document removal.
Org syntax assumes planning info line is always right below the
headline. Setting this variable to anything but nil defeats that and,
as a consequence, produces invalid planning lines.