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

lisp/org-num.el

@@ -461,6 +461,7 @@ NUMBERING is a list of numbers."
    (org-num-mode
     (unless (derived-mode-p 'org-mode)
       (user-error "Cannot activate headline numbering outside Org mode"))
+    (org-num--clear)
     (setq org-num--numbering nil)
     (setq org-num--overlays (nreverse (org-num--number-region nil nil)))
     (add-hook 'after-change-functions #'org-num--verify nil t)