Remove deprecated Legacy SidekiqMemoryKiller (The middleware)

In #232765 (closed) we enabled daemon memory killer to be the default one.

Legacy memory killer was deprecated 2 years ago.

We should remove the legacy memory killer in the long term, because:

• legacy memory killer is expected to be replaced by daemon memory killer
• maintain two memory killers requires extra effort
• maintain two memory killers makes configuration complex, sometimes it is confusing when diagnosing issues, one example is: https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10930#note_386967266