Remove unnecessary defined('ENVIRONMENT') checks

As suggested in issue #2134 & PR #2149
diff --git a/system/core/Loader.php b/system/core/Loader.php
index 1ad07f1..bbd7a84 100644
--- a/system/core/Loader.php
+++ b/system/core/Loader.php
@@ -1089,12 +1089,12 @@
 					// We test for both uppercase and lowercase, for servers that
 					// are case-sensitive with regard to file names. Check for environment
 					// first, global next
-					if (defined('ENVIRONMENT') && file_exists($path.'config/'.ENVIRONMENT.'/'.strtolower($class).'.php'))
+					if (file_exists($path.'config/'.ENVIRONMENT.'/'.strtolower($class).'.php'))
 					{
 						include($path.'config/'.ENVIRONMENT.'/'.strtolower($class).'.php');
 						break;
 					}
-					elseif (defined('ENVIRONMENT') && file_exists($path.'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php'))
+					elseif (file_exists($path.'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php'))
 					{
 						include($path.'config/'.ENVIRONMENT.'/'.ucfirst(strtolower($class)).'.php');
 						break;
@@ -1180,7 +1180,7 @@
 	 */
 	protected function _ci_autoloader()
 	{
-		if (defined('ENVIRONMENT') && file_exists(APPPATH.'config/'.ENVIRONMENT.'/autoload.php'))
+		if (file_exists(APPPATH.'config/'.ENVIRONMENT.'/autoload.php'))
 		{
 			include(APPPATH.'config/'.ENVIRONMENT.'/autoload.php');
 		}