Remove redundant "$thing instanceof \Countable" check

The check L1183 is redundant as "$thing instanceof \Countable" is already checked L1175.

```php
        if ($thing instanceof \Countable || \is_array($thing) || $thing instanceof \SimpleXMLElement) {
            return \count($thing);
        }

        if ($thing instanceof \Traversable) {
            return iterator_count($thing);
        }

        if (method_exists($thing, '__toString') && !$thing instanceof \Countable) {
            return mb_strlen((string) $thing, $env->getCharset());
        }
```

src/Extension/CoreExtension.php

@@ -1180,7 +1180,7 @@ final class CoreExtension extends AbstractExtension
             return iterator_count($thing);
         }
 
-        if (method_exists($thing, '__toString') && !$thing instanceof \Countable) {
+        if (method_exists($thing, '__toString')) {
             return mb_strlen((string) $thing, $env->getCharset());
         }