minor #42597 [Form] Remove deprecated code from FormErrorIterator (W0rma)

This PR was merged into the 6.0 branch.

Discussion
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[Form] Remove deprecated code from FormErrorIterator

| Q             | A
| ------------- | ---
| Branch?       | 6.0
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Tickets       | Follow-up of symfony/symfony#42387
| License       | MIT
| Doc PR        |

Remove deprecated code from `FormErrorIterator::children()`

Commits
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07986234cc Throw exception if child element is not iterable

Author: nicolas-grekas

FormErrorIterator.php

@@ -186,8 +186,7 @@ public function hasChildren(): bool
     public function getChildren(): self
     {
         if (!$this->hasChildren()) {
-            trigger_deprecation('symfony/form', '5.4', 'Calling "%s()" if the current element is not iterable is deprecated, call "%s" to get the current element.', __METHOD__, self::class.'::current()');
-            // throw new LogicException(sprintf('The current element is not iterable. Use "%s" to get the current element.', self::class.'::current()'));
+            throw new LogicException(sprintf('The current element is not iterable. Use "%s" to get the current element.', self::class.'::current()'));
         }
 
         return current($this->errors);