Remove unnecessary defined('ENVIRONMENT') checks

As suggested in issue #2134 & PR #2149
diff --git a/system/core/CodeIgniter.php b/system/core/CodeIgniter.php
index 8affde6..cb4b735 100644
--- a/system/core/CodeIgniter.php
+++ b/system/core/CodeIgniter.php
@@ -58,7 +58,7 @@
  *  Load the framework constants
  * ------------------------------------------------------
  */
-	if (defined('ENVIRONMENT') && file_exists(APPPATH.'config/'.ENVIRONMENT.'/constants.php'))
+	if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/constants.php'))
 	{
 		require(APPPATH.'config/'.ENVIRONMENT.'/constants.php');
 	}