Revert decoding of past block types

[sideninja]

We need to revert the logic for decoding previous types of blocks. During development block type was changed (new fields were added), and because blocks were persisted on previewnet we have to handle decoding different types of blocks, but once previewnet is gone there's no other chain persisting previous types of blocks so decoding logic can be removed.

This code needs to be removed: https://github.com/onflow/flow-go/blob/master/fvm/evm/types/block.go#L198-L267