lint: remove unused requires

duh.

lib/application.js

@@ -11,7 +11,6 @@ var response = require('./response');
 var Cookies = require('cookies');
 var Keygrip = require('keygrip');
 var assert = require('assert');
-var Stream = require('stream');
 var http = require('http');
 var co = require('co');
 

lib/response.js

@@ -5,7 +5,6 @@
 
 var debug = require('debug')('koa:response');
 var statuses = require('./status');
-var Stream = require('stream');
 var http = require('http');
 var path = require('path');
 var mime = require('mime');