chore(xo-server-backup-reports): removing unused legacy handler

[b-Nollet]

Description

Removing _legacyVmHandler from the backup reports plugin, which was used for legacy backups and has apparently become useless.

Checklist

• Commit
  • Title follows commit conventions
  • Reference the relevant issue (Fixes #007, See xoa-support#42, See https://...)
  • If bug fix, add Introduced by
• Changelog
  • If visible by XOA users, add changelog entry
  • Update "Packages to release" in CHANGELOG.unreleased.md
• PR
  • If UI changes, add screenshots
  • If not finished or not tested, open as Draft

[b-Nollet]

don't forget the changelog

Do we need to write in the changelog the changes that have no impact for users ?

[MathieuRA]

Do we need to write in the changelog the changes that have no impact for users ?

No, BTW you must add xo-server-backup-reports in the Packages to release section