org-attach: Fix :type for org-attach-use-inheritance

* lisp/org-attach.el (org-attach-use-inheritance): Drop repeated :type
keyword that incorrectly marked the option as boolean.

Reported-by: Philip Blagoveschensky <philip@crabman.me>
https://orgmode.org/list/7036d668-eb09-ba4d-0b46-503fbc4a50ce@crabman.me

lisp/org-attach.el

@@ -128,8 +128,7 @@ Selective means to respect the inheritance setting in
   :type '(choice
 	  (const :tag "Don't use inheritance" nil)
 	  (const :tag "Inherit parent node attachments" t)
-	  (const :tag "Respect org-use-property-inheritance" selective))
-  :type 'boolean)
+	  (const :tag "Respect org-use-property-inheritance" selective)))
 
 (defcustom org-attach-store-link-p nil
   "Non-nil means store a link to a file when attaching it."