[Form] Revert "Fix "Array was modified outside object" in ResizeFormListener."

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10256, #10263
| License       | MIT
| Doc PR        | 

Unseting key on iterator is not an solution because in php "Array assignment always involves value copying. " So if \IteratorAggregate implementation create new iterator from array (just like [Doctrine ArrayCollection](https://github.com/doctrine/common/blob/2.3/lib/Doctrine/Common/Collections/ArrayCollection.php#L362) does)  such operation will not affect original data. 

I'm just not sure if it's good to use Doctirn ArrayCollection in this test case or maybe its better to create own implementation of such Collection as a fixture.