Remove latest_execution_payload_header from the beacon state

[potuz]

The Beacon State tracks the latest execution payload header in full, but the only thing that this is used for is to check that the parent hash is the right one. We should only track the latest block hash instead.

Every change in the execution header forces clients to keep these changes on the beacon state and track new structures. There have been numerous marshalling bugs because of endinannes, large number libraries need to be imported and maintained up to date on the CL without any need since these structures should be opaque as they are not used on the CL.