Remove SER_GETHASH, hard-code client version in CKeyPool serialize

Summary:
It was never set, so it can be removed along with any code reading it.

This concludes backport of core#28508
https://github.com/bitcoin/bitcoin/pull/28508/commits/fac29a0ab19fda457b55d7a0a37c5cd3d9680f82
Depends on D19144

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19145