Remove unnecessary defined('ENVIRONMENT') checks

As suggested in issue #2134 & PR #2149
diff --git a/system/core/Router.php b/system/core/Router.php
index f284e29..4755b37 100644
--- a/system/core/Router.php
+++ b/system/core/Router.php
@@ -133,7 +133,7 @@
 		}
 
 		// Load the routes.php file.
-		if (defined('ENVIRONMENT') && is_file(APPPATH.'config/'.ENVIRONMENT.'/routes.php'))
+		if (is_file(APPPATH.'config/'.ENVIRONMENT.'/routes.php'))
 		{
 			include(APPPATH.'config/'.ENVIRONMENT.'/routes.php');
 		}