Remove unnecessary defined('ENVIRONMENT') checks

As suggested in issue #2134 & PR #2149
diff --git a/system/core/Hooks.php b/system/core/Hooks.php
index 2cb416c..59759e0 100644
--- a/system/core/Hooks.php
+++ b/system/core/Hooks.php
@@ -81,7 +81,7 @@
 		}
 
 		// Grab the "hooks" definition file.
-		if (defined('ENVIRONMENT') && is_file(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
+		if (is_file(APPPATH.'config/'.ENVIRONMENT.'/hooks.php'))
 		{
 			include(APPPATH.'config/'.ENVIRONMENT.'/hooks.php');
 		}