Remove unused require

Author: andreypopp

lib/RouterMixin.js

@@ -2,7 +2,6 @@
 
 var React         = require('react');
 var invariant     = require('react/lib/invariant');
-var emptyFunction = require('react/lib/emptyFunction');
 var matchRoutes   = require('./matchRoutes');
 var Environment   = require('./environment');