EVM/Kernel: fix BLOCKHASH behaviour

Context

Fix #6985 (closed)

As of right now the BLOCKHASH opcode always return 0. That's actually normal since we're trying to read in the storage at a location where nothing is written at all. This MR fixes that.

The first commit contains some simplification (basically removing dead code).

Manually testing the MR

I've added a simple tezt test with a solidity contract to check that what we retrieve from the RPC by decoding the entire block is the same as what the BLOCKHASH opcode returns.

dune exec tezt/tests/main.exe -- --file evm_rollup.ml evm blockhash opcode 

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR