refactor(cketh/ckerc20): use consistently LedgerSubaccount

[gregorydemay]

(XC-221): LedgerSubaccount was introduced with #2151 to be used for events stored in stable memory and was used in particular for events related to deposits. Unfortunately, withdrawal events were already prepared to received subaccounts, but they used a different type with a different, less efficient, encoding. This PR removes the old type to only keep LedgerSubaccount, which is in preparation of supporting subaccounts for withdrawals.

Remarks:

1. There are already instances of ReceivedEthEvent and ReceivedErc20Event with some LedgerSubaccount value on the ckSepoliaETH minter.
2. All other events EthWithdrawalRequest, Erc20WithdrawalRequest, and ReimbursementRequest that used the old type for subaccount only had instances on the ckETH and ckSepoliaETH minter that had None for value. This allows to use LedgerSubaccount for those events in a backwards-compatible manner.