Revert "Merge branch 'show-issue-ownership-in-service-desk' into 'master'"

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR reverts another that caused a regression in the behavior of Service Desk.

The external_author field on Issues aliases service_desk_reply_to and should always be an email address. It is used in NotificationService.send_service_desk_notification to avoid notifying the original author of an issue when new comments come in from themselves.

The user experience is that authors now have their own comments echoed back to themselves by email.

The MR reverted is !81207 (merged)

Reverting this MR will not be enough to prevent the issue continuing with Service Desk issues created while the code was in production but will prevent further issues being created with this bug. It's an annoyance rather than a broken feature and a migration can be written in future to repair the broken external_author field.

See https://gitlab.com/gitlab-org/gitlab/-/issues/355761

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.