[Form] Fix items not being deleted from collection

``ArrayIterator::offsetUnset()`` doesn't unset on the original array so the result needs to be reassigned. Fixes #10256.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #10256
| License       | MIT
| Doc PR        |