Fix `org-copy-visible'

* lisp/org.el (org-copy-visible): Deactivate region before adding
  visible part to kill ring.

See https://lists.gnu.org/archive/html/emacs-devel/2018-09/msg00476.html

lisp/org.el

@@ -20062,6 +20062,7 @@ this numeric value."
       (let ((next (next-single-char-property-change beg 'invisible nil end)))
 	(setq result (concat result (buffer-substring beg next)))
 	(setq beg next)))
+    (setq deactivate-mark t)
     (kill-new result)
     (message "Visible strings have been copied to the kill ring.")))