Remove FF_RESET_HELPER_IMAGE_ENTRYPOINT feature flag

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. Once we know that the entrypoint works as expected and users don't run into issues requiring to turn this FF off we should remove this feature flag in %14.0