I have the following url..
http://localhost/ci/site_controller/home
I want to remove site_controller controller
assuming you currently have your url http://localhost/ci/site_controller/home you can just write an explicit route like below to your /application/config/routes.php
$route['ci/home'] = 'site_controller/home';
or
$route['ci/home'] = 'ci/site_controller/home';
if your controller is namespaced to /application/controllers/ci/