Remove PHP 5.1.6-specific code
diff --git a/system/core/URI.php b/system/core/URI.php
index db5b8e4..4a2e87c 100755
--- a/system/core/URI.php
+++ b/system/core/URI.php
@@ -444,9 +444,7 @@
 				return array();
 			}
 
-			return function_exists('array_fill_keys')
-				? array_fill_keys($default, FALSE)
-				: array_combine($default, array_fill(0, count($default), FALSE));
+			return array_fill_keys($default, FALSE);
 		}
 
 		$segments = array_slice($this->$segment_array(), ($n - 1));