[net processing] Remove CNodeState.nBlocksInFlightValidHeaders

Summary:
nBlocksInFlightValidHeaders always has the same value as nBlocksInFlight, since we only download
blocks with valid headers.

This is a partial backport of core#22141
https://github.com/bitcoin/bitcoin/pull/22141/commits/b03de9c7538d85b12929a62b9ec966fd3910e660

Depends on D12703

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12704