remove dead code in multiprocess_validation

[arvidn]

Purpose:

We never pass HeaderBlock objects to multiprocess_validation(), only FullBlocks. Remove all code that handles HeaderBlocks in the multiprocess_validation path

This is best viewed ignoring whitespace changes, since there's a large block that got de-dented.

Current Behavior:

New Behavior:

Testing Notes:

[github-actions]

File	Coverage	Missing Lines
chia/consensus/multiprocess_validation.py	89.1%	lines 81, 148-152

Total	Missing	Coverage
55 lines	6 lines	89%

[arvidn]

I think it's reasonable to ignore the missing test coverage. It's pre-existing and it's in the case of validation failure.

[almogdepaz]

lgtm