Etherlink/Kernel: don't store gas price out of blocks

What

Continuation of !15574 (merged) in which we stop storing the base fee independently at path /evm/world_state/base_fee_per_gas and instead rely on the fact that it is already stored in the current block.

Why

To save the performance of a storage write (at the price of the indirection when we need to read the information) and to simplify the code.

How

Manually testing the MR

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