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