[NodeTypeResolver] Clean up mixed type check on TypeFactory::uniquateTypes() (#7351)

Author: samsonasik

src/NodeTypeResolver/PHPStan/Type/TypeFactory.php

@@ -64,10 +64,6 @@ public function uniquateTypes(array $types, bool $keepConstant = false): array
         $hasFalse = false;
         $hasTrue = false;
         foreach ($types as $type) {
-            if ($type instanceof MixedType) {
-                $type = new MixedType();
-            }
-
             $type = $this->normalizeObjectType($totalTypes, $type);
             $type = $this->normalizeBooleanType($hasFalse, $hasTrue, $type);