0b1f4b3c6685d0a6307926d43d166add538061b7 wallet: Drop unused OldKey (João Barbosa)
Pull request description:
Replaces #16494, `OldKey` (previously `CWalletKey`) was never serialized in the code history which means that unserialization support is not required, so remove the code entirely.
https://github.com/bitcoin/bitcoin/pull/16502/commits/0b1f4b3c6685d0a6307926d43d166add538061b7
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Depends on D7022
Backport of Core PR16502