Remove old authentication header for call made by agentk to GitLab

MR:

• Remove Gitlab-Agentk-Api-Request header (!2697 - merged)
• Remove Gitlab-Agentk-Api-Request header (gitlab-org/gitlab!199392 - merged)

Description

In Add new authentication header for calls made by... (#710 - closed) , we added new authentication header. Once changes have been rolled out, we should delete the old authentication header.

Acceptance criteria

• Remove the old authentication header Gitlab-Agentk-Api-Request in both Rails and GitLab Agent codebase.