Remove unnecessary defined('ENVIRONMENT') checks

As suggested in issue #2134 & PR #2149
diff --git a/system/helpers/html_helper.php b/system/helpers/html_helper.php
index 2d47416..7a71eb8 100644
--- a/system/helpers/html_helper.php
+++ b/system/helpers/html_helper.php
@@ -242,7 +242,7 @@
 
 		if ( ! is_array($_doctypes))
 		{
-			if (defined('ENVIRONMENT') && is_file(APPPATH.'config/'.ENVIRONMENT.'/doctypes.php'))
+			if (is_file(APPPATH.'config/'.ENVIRONMENT.'/doctypes.php'))
 			{
 				include(APPPATH.'config/'.ENVIRONMENT.'/doctypes.php');
 			}