Remove outdated backup command from registry Makefile

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What does this merge request do and why?

Removes automatic backup of container-registry during GDK updates to prevent the .backups folder from growing to several GB.

Solves issue#2418

How to set up and validate locally

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

1. Set up and start the registry

   make registry-setup
   bundle exec ./bin/gdk start

2. Verify no backups created:

    ls -la .backups/
    bundle exec ./bin/gdk update
    ls -la .backups/
    curl -I http://registry.test:5100/v2/

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.