ethrex: remove lost keccak calculation in guest program

[jsign]

This is a bit unfortunate since we're discarding a keccak calculation.

The reason for this is here.

The received blocks must not have their hash already calculated. I suspect this is because they want to be strict on not received precomputed hashes as inputs.

For another angle, here in L15 we do let block_hash = input.blocks[0].hash(); which indirectly caches the calculated hash, thus the provided block does not meet the expectation later inside the block validation logic (linked above).

I think ideally Ethrex should accept the hashes value to be set, but always compute it in the validation and compare it (if present). This would make things a bit more flexible here, while keeping the "do not trust" values.

Note that the reason why I want the hash is to do the L23 check, so in the case of wanting to prove more than one block I can still commit to the block hash. I could simply avoid that expectation and let the host see that the third output (i.e., successful block validation) was false anyway, but feels like an unnecessary compromise to workaround this.

I also thought about doing the .len() != 1 after the execution_program, but that's a bit messy since res is a Result, plus I can't do input.blocks[0] since input was moved in execution_program(input) call.

We can live with this for a while, but eventually would be nice to remove this workaround.

Originally posted by @jsign in #176 (comment)