Do not save to disk whether UTXOs are locked.

The lockunspent RPC is volatile, that is, it only locks unspent
transaction outputs from being used as inputs for the duration of the
wallet process, or until the UTXO is unlocked with a later call to
lockunspent.  Therefore, remove the serialization of the lockedness
when writing txstore Credits.

The space which used to contain the locked flag is now unused and may
be used for other flags in the future.