fix: remove unserialize() in old()

I can't think of any use cases.

system/Common.php

@@ -813,11 +813,6 @@ if (! function_exists('old')) {
             return $default;
         }
 
-        // If the result was serialized array or string, then unserialize it for use...
-        if (is_string($value) && (strpos($value, 'a:') === 0 || strpos($value, 's:') === 0)) {
-            $value = unserialize($value);
-        }
-
         return $escape === false ? $value : esc($value, $escape);
     }
 }

tests/system/CommonFunctionsTest.php

@@ -300,7 +300,7 @@ final class CommonFunctionsTest extends CIUnitTestCase
         $_GET     = ['foo' => 'bar'];
         $_POST    = [
             'bar'    => 'baz',
-            'zibble' => serialize('fritz'),
+            'zibble' => 'fritz',
         ];
 
         $response = new RedirectResponse(new App());