validation: deduplicate forward declaration of Consensus::Params

Summary:
This was duplicated in D13244, when backporting https://github.com/bitcoin/bitcoin/pull/24595/commits/69675ea4e73dcf5e9dd0f94802bd3463e4262081

We first added it in D4033

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16259