Some cleanup related to mt_rand()

- min and max values are 0 and mt_getrandmax() by default
- remove useless mt_srand() seed calls
diff --git a/system/libraries/Session/drivers/Session_cookie.php b/system/libraries/Session/drivers/Session_cookie.php
index 0e86441..7174d63 100644
--- a/system/libraries/Session/drivers/Session_cookie.php
+++ b/system/libraries/Session/drivers/Session_cookie.php
@@ -641,7 +641,7 @@
 		$new_sessid = '';
 		do
 		{
-			$new_sessid .= mt_rand(0, mt_getrandmax());
+			$new_sessid .= mt_rand();
 		}
 		while (strlen($new_sessid) < 32);
 
@@ -832,7 +832,6 @@
 		$probability = ini_get('session.gc_probability');
 		$divisor = ini_get('session.gc_divisor');
 
-		srand(time());
 		if ((mt_rand(0, $divisor) / $divisor) < $probability)
 		{
 			$expire = $this->now - $this->sess_expiration;