remove a few redundant methods for header field values

lib/context.js

@@ -924,32 +924,6 @@ Context.prototype = {
     }
   },
 
-  setHeader: function(field, val){
-    this.set(field, val);
-  },
-
-  /**
-   * Get the current response header `name`.
-   *
-   * @param {String} name
-   * @api public
-   */
-
-  getHeader: function(name){
-    return this.res.getHeader(name);
-  },
-
-  /**
-   * Remove the current response header `name`.
-   *
-   * @param {String} name
-   * @api public
-   */
-
-  removeHeader: function(name){
-    return this.res.removeHeader(name);
-  },
-
   /**
    * Get the trailing headers to a request.
    *