Declare `org-usenet-links-prefer-google' as obsolete

* lisp/org-compat.el (org-usenet-links-prefer-google): Declare as
  obsolete.

lisp/org-compat.el

@@ -365,6 +365,9 @@ use of this function is for the stuck project list."
 (define-obsolete-function-alias 'org-babel-number-p
   'org-babel--string-to-number "Org 9.0")
 
+(define-obsolete-variable-alias 'org-usenet-links-prefer-google
+  'org-gnus-prefer-web-links "Org 9.1")
+
 
 ;;;; Obsolete link types
 

lisp/org-gnus.el

@@ -43,8 +43,6 @@
 
 ;;; Customization variables
 
-(defvaralias 'org-usenet-links-prefer-google 'org-gnus-prefer-web-links)
-
 (defcustom org-gnus-prefer-web-links nil
   "If non-nil, `org-store-link' creates web links to Google groups or Gmane.
 \\<org-mode-map>When nil, Gnus will be used for such links.