[feature]: Remove Endorsement experiment code

[ziggie1984]

The endorsement period ended at January 1, 2026 12:00:00 AM UTC. Now we do not forward any endorsement bit anymore. So it is time to update our code to the final proposal and remove the experiment harness from the codebase.

We already fixed the itests which were failing due to the ending of the signal period so we also in the best case make the test independant of this as well.

See also:

#10476,

#10367 (comment)

[carlaKC]

So it is time to update our code to the final proposal

I don't think that we're quite there yet? Typically we only update to use non-experimental feature bits once the BOLT PR has been merged.

The idea of an expiry date in the original proposal was:

1. To mitigate any privacy concerns (which no longer apply, with our updated proposal to have a default value of 0)
2. To stop relaying the signal in the case where we're not going to proceed with this proposal

Do you have objections to leaving relay of the experimental feature bit (without time restrictions, as done in #10367) until the BOLT PR is merged? As mentioned the experimental bit continues to be useful to ongoing work.

[ziggie1984]

Do you have objections to leaving relay of the experimental feature bit (without time restrictions, as done in #10367) until the BOLT PR is merged? As #10367 (comment) the experimental bit continues to be useful to ongoing work.

Sorry I think I misunderstood the PR in #10367 (after the update yesterday), now I see that the EndorsementExperimentEnd was not just renamed but removed. So I think we can close this issue as a consequence.