validation: Remove global ::LoadGenesisBlock

Summary:
This is a backport of core#21055 [2/16] and core#21270 [1/12]
https://github.com/bitcoin/bitcoin/pull/21055/commits/4927c9e6991b09a36a41aab93a0e05332d899611
https://github.com/bitcoin/bitcoin/pull/21270/commits/a04aac493fd564894166d58ed4cdfd9ad4f561cb

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11206