EVM/Execution: remove ExecutionOutcome.is_success

What

ExecutionOutcome refactor first step

Related to #6664 (closed)

Why

The plan is to replace ExecutionOutcome.{is_success, reason, new_address, result} by one coherent enum as stated in #6664 (closed). Before introducing the new enum, we first remove is_success to make the refactor less cumbersome.

How

the field reason is enough to know if the execution succeeded.

Manually testing the MR

In /etherlink/kernel_evm:

make test

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