refactor: Remove unused validation includes

Summary:

Unused includes will cause needless recompilation when headers are changed. Also, they pretend there are dependencies that don't exist.

Fix both by removing them.

This is a backport of core#23044

Notes:

• the CScriptCheck forward declaration is needed by Bitcoin ABC, because the CheckScriptInputs function is declared in validation.h rather than validation.cpp
• I removed also two unused includes in validation.cpp flagged by my IDE

Test Plan: ninja all check-all bitcoin-fuzzers bench-bitcoin

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12697