Simplifying timelock's environment

Context

Timelock opening_result type can be simplified as we do not deal with Bogus_cipher any longer. This change however affect the protocol environment, and so was postponed to a different MR (than the re enabling of timelock).

This is the goal of the MR.

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 MRDrqf