fix(auth): remove extra bson include

Author: mbroadst

lib/auth/plain.js

@@ -1,7 +1,5 @@
 "use strict";
 
-var BSON = require('bson');
-
 var f = require('util').format
   , retrieveBSON = require('../connection/utils').retrieveBSON
   , Query = require('../connection/commands').Query