refactor: Remove unused CExt{Pub,}Key (de)serialization methods

Summary:
The serialization/deserialization methods for the classes CExtKey and
CExtPubKey were only used in the BIP32 unit tests, where the relevant parts are
removed as well.

Backport of Bitcoin Core PR17212

Test Plan: ninja check

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7553