Remove unnecessary leading namespace separator.

As it currently stands, global namespace classes are imported like so:

```php
use \PDO;            // global namespace import
use Vendor\Package;  // non-global namespace import
```

But leading namespace separators are meaningless in use statements, and it breaks consistency with non-global imports.

This PR brings global namespace imports inline with the others, like so:

```php
use PDO;             // global namespace import
use Vendor\Package;  // non-global namespace import
```