[Sets] Remove deprecated PHPUnitSetProvider

[TomasVotruba]

SetProviderInterface, Set and the whole set-object machinery are deprecated in rectorphp/rector-src#8296 — rules now carry their own version info via ComposerPackageConstraintInterface.

Here the provider was already dead weight. Since #757 and #758 it returned plain Set objects only, and SetManager::matchBySetGroups() matches ComposerTriggeredSet exclusively:

foreach ($this->setProviderCollector->provideComposerTriggeredSets() as $composerTriggeredSet) {

So ->withSetGroups(SetGroup::PHPUNIT) loaded nothing from it. It is also absent from core's SetProviderCollector list (PHP, Core, Doctrine, Twig).

-final class PHPUnitSetProvider implements SetProviderInterface
-{
-    public function provide(): array
-    {
-        return [
-            new Set(SetGroup::PHPUNIT, 'Composer Based', __DIR__ . '/../../../config/sets/composer-based.php'),
-            new Set(SetGroup::PHPUNIT, 'Code Quality', __DIR__ . '/../../../config/sets/phpunit-code-quality.php'),
-            new Set(SetGroup::ATTRIBUTES, 'PHPUnit Attributes', __DIR__ . '/../../../config/sets/annotations-to-attributes.php'),
-        ];
-    }
-}

Sets stay reachable as before:

$rectorConfig->withSets([PHPUnitSetList::COMPOSER_BASED]);

No other reference to the class existed in configs, docs or tests.