Remove CHashWriter type

Summary:
The type is only ever set, but never read via GetType(), so remove it.
Also, remove SerializeHash to avoid silent merge conflicts and use the
already existing GetHash() boilerplate consistently.

This is a partial backport of core#28508
https://github.com/bitcoin/bitcoin/pull/28508/commits/fa72f09d6ff8ee204f331a69d3f5e825223c9e11
https://github.com/bitcoin/bitcoin/pull/28341/commits/99995cfe8da6ea2b93a6cd0e0bc84bb34cbb9d8c (addrman.cpp callsite missed in D16472 because we didn't backport core#22910)
Depends on D19143

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19144