[validation] Remove unused first_invalid parameter from ProcessNewBlockHeaders()

Summary:
No callers use the returned value in first_invalid. Remove it from the
function signature and don't set it in the function.

Partial backport of Core PR15921 : https://github.com/bitcoin/bitcoin/pull/15921/commits/c428622a5bb1e37b2e6ab2c52791ac05d9271238

Depends on D6873 only to be lint-free.

Test Plan: ninja check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6874