remove the vary method from request object

docs/api/request.md

@@ -315,10 +315,6 @@ this.acceptsLanguages();
 
   Return the request socket.
 
-### req.vary(field)
-
-  Vary on `field`.
-  
 ### req.get(field)
 
   Return request header.

lib/request.js

@@ -459,17 +459,6 @@ module.exports = {
     return this.accept.languages.apply(this.accept, arguments);
   },
 
-  /**
-   * Vary on `field`.
-   *
-   * @param {String} field
-   * @api public
-   */
-
-  vary: function(field){
-    this.append('Vary', field);
-  },
-
   /**
    * Check if the incoming request contains the "Content-Type"
    * header field, and it contains any of the give mime `type`s.