[backport#16502] wallet: Drop unused OldKey
Summary:
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
Depends on D7022
Backport of Core PR16502
Test Plan:
ninja check check-functional
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Subscribers: Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7023