ocex withdrawal storage keys not freed after claim.

After a user claims a withdrawal in ocex the storage key is not pruned after the value is deleted. so the storage is left with alot of keys with just empty values. This will result in a continuously increasing storage.

<img width="1145" alt="image" src="https://github.com/Polkadex-Substrate/Polkadex/assets/50878033/6afab45c-e698-4804-b5e0-50d8e077a898">


