Remove FF_RESET_HELPER_IMAGE_ENTRYPOINT feature flag

What does this MR do?

Removes the FF_RESET_HELPER_IMAGE_ENTRYPOINT feature flag

Why was this MR needed?

In !2058 (merged) we started resetting the ENTRYPOINT metadata for automatically loaded helper image tar archives, which lose that information when exported to a tar archive. We introduced FF_RESET_HELPER_IMAGE_ENTRYPOINT to allow customers which use their own helper image archives to go back to the old behaviour in case they don't expose a /entrypoint file. Now that we know that the entrypoint works as expected and users don't run into issues requiring to turn this FF off we will remove this feature flag as part of the %14.0 cycle of deprecations.

What's the best way to test this MR?

To test that the ENTRYPOINT is preserved, we can take advantage of the existing helpers/entrypoint logic and introduce a path to an invalid file in CA_CERTIFICATES_PATH. The script will try to import it as a CA but will fail. The failure error in the build log proves that the entrypoint was correctly assigned to the helper image.

1. Build runner:

   make runner-bin-host

2. Register a docker runner:

   gitlab-runner register --config /tmp/config.toml \
       --non-interactive \
       --executor "docker" \
       --docker-image "alpine:latest" \
       --url "https://gitlab.com" \
       --description "Docker Runner #26679" \
       --tag-list="26679" \
       --registration-token "$TOKEN"

3. Run a job tagged to the runner just registered:

   gitlab-runner run --config /tmp/config.toml

   start_evaluation:
     variables:
       CA_CERTIFICATES_PATH: /usr/bin/beep # Use existing path to an invalid file
     script:
       - date
     tags: ["26679"]

4. The output log should contain message about CA import:

   Updating CA certificates...
   WARNING: ca-certificates.crt does not contain exactly one certificate or CRL: skipping
   WARNING: ca-cert-ca.pem does not contain exactly one certificate or CRL: skipping

   Example: https://gitlab.com/pedropombeiro/playground/-/jobs/1282850590

5. Cleanup:

   gitlab-runner unregister --all-runners --config /tmp/config.toml

What are the relevant issue numbers?

Closes #26679 (closed)