Merge #12128: Refactor: One CBaseChainParams should be enough

Summary:
1687cb4 Refactor: One CBaseChainParams should be enough (Jorge Timón)

Pull request description:

There's no need for class hierarchy with CBaseChainParams, it is just a struct with 2 fields.
This starts as a +10-43 diff

Tree-SHA512: 0a7dd64ab785416550b541787c6083540e4962d76b6cffa806bb3593aec2daf1752dfe65ac5cd51b34ad5c31dd8292c422b483fdd2d37d0b7e68725498ed4c2d

Backport of Core PR12128
https://github.com/bitcoin/bitcoin/pull/12128/

Test Plan:

make check

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D3804