[Sets] Remove the empty Symfony per-version set providers

[TomasVotruba]

Final step of the set cleanup started in #1010.

Those six providers registered 208 per-version ComposerTriggeredSets, all of them already covered rule for rule by config/sets/symfony/composer-based.php, where each rule carries its own package version constraint. #1010 emptied them, keeping only the class shells because rector-src instantiated them by name. rectorphp/rector-src#8292 removed that reference, so what is left is six classes like this:

final class Symfony8SetProvider implements SetProviderInterface
{
    /**
     * @return SetInterface[]
     */
    public function provide(): array
    {
        return [];
    }
}

All six go. TwigSetProvider stays — SetGroup::TWIG is wired into withComposerBased(), so its composer-based trigger is still resolved through SetManager, and it is the only provider rector-src still instantiates from this package.

The per-version set files are untouched and still reachable through the SymfonySetList::SYMFONY_* constants.