org-num-mode: Clear remaining num overlays before activation
* lisp/org-num.el (org-num-mode): Call `org-num--clear' when enabling
the mode. This avoids duplicate overlays when the mode was active
prior (org-num-mode +1) call.
Reported-by: Ruijie Yu
Link: https://orgmode.org/list/sdvbkq8a7lp.fsf@fw.net.yu