[DeadCode] Move RemoveReadonlyPropertyVisibilityOnReadonlyClassRector to dead-code set

[TomasVotruba]

The rule removes redundant visibility-less readonly noise on readonly classes, where the property readonly modifier is already implied. That is dead code removal rather than code quality, so it fits the dead-code set better.

 final readonly class SomeClass
 {
-    public readonly string $name;
+    public string $name;

-    public function __construct(private readonly int $age)
+    public function __construct(private int $age)
     {
     }
 }

The rule class stays in the CodeQuality namespace; only the set registration moves from CodeQualityLevel to DeadCodeLevel.