[Validator] Fixed memory leak in ValidatorBuilder

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11236
| License       | MIT
| Doc PR        | -

In 23534ca6ab4a107a5e801e710fb3c07b1a8874d9, the following code was introduced in `ValidatorBuilder::getValidator()`:

```php
AnnotationRegistry::registerLoader(function ($class) {
    if (0 === strpos($class, __NAMESPACE__.'\\Constraints\\')) {
        $file = str_replace(__NAMESPACE__.'\\Constraints\\', __DIR__.'/Constraints/', $class).'.php';

        if (is_file($file)) {
            require_once $file;

            return true;
        }
    }

    return false;
});
```

`AnnotationRegistry::registerLoader()` stores all loaders in a global array. Every time that `getValidator()` is called, a new closure is put onto the loader stack, referencing `$this` (i.e. the ValidatorBuilder) and consequently preventing the ValidatorBuilder (and its references) from being garbage-collected.

The call to `registerLoader()` did not exist in 2.4 and I can't find a reason why it should be there. All tests are green without that code. I suppose I used it for debugging and then forgot to remove it again, so I'm removing it here.