Make use of the constant `org-clock-string' whenever possible

Instead of the hardcoded value "CLOCK:".

* lisp/org-clock.el (org-find-open-clocks)
* lisp/org.el (org-clone-subtree-with-time-shift)
* lisp/org.el (org-insert-property-drawer)
* lisp/org.el (org-at-clock-log-p)

TINYCHANGE