[net processing] Remove QueuedBlock.fValidatedHeaders

Summary:
Since headers-first syncing, we only ever request a block if we've already validated its headers.
Therefore QueuedBlock.fValidatedHeaders is always set to true. Remove it.

This is a partial backport of core#22141
https://github.com/bitcoin/bitcoin/pull/22141/commits/b4e29f2436943c131dd25b123d13a25ce09bab58

Depends on D12702

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D12703