[6.3] Remove unused test fixture

Author: fancyweb

Tests/ParameterBagTest.php

@@ -379,15 +379,3 @@ public function testGetEnumThrowsExceptionWithInvalidValueType()
         $this->assertNull($bag->getEnum('invalid-value', FooEnum::class));
     }
 }
-
-class InputStringable
-{
-    public function __construct(private string $value)
-    {
-    }
-
-    public function __toString(): string
-    {
-        return $this->value;
-    }
-}