Remove unnecessary defined('ENVIRONMENT') checks

As suggested in issue #2134 & PR #2149
diff --git a/system/database/DB.php b/system/database/DB.php
index f94685c..d910474 100644
--- a/system/database/DB.php
+++ b/system/database/DB.php
@@ -43,7 +43,7 @@
 	if (is_string($params) && strpos($params, '://') === FALSE)
 	{
 		// Is the config file in the environment folder?
-		if (( ! defined('ENVIRONMENT') OR ! file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/database.php'))
+		if ( ! file_exists($file_path = APPPATH.'config/'.ENVIRONMENT.'/database.php')
 			&& ! file_exists($file_path = APPPATH.'config/database.php'))
 		{
 			show_error('The configuration file database.php does not exist.');