Etherlink/Kernel: remove unused __at_most_one_block flag

What

This MR removes the __at_most_one_block option of the EVM kernel which allows limiting the application of blocks to at most one per L1 level. This was introduced in !13202 (merged) but is not used anymore in the EVM node (according to git grep).

Why

To simplify block::produce even more than previous MRs (!16274 (merged) and !16279 (merged)) on which this MR is based.

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