Remove rake setup from diff-config command

What does this merge request do and why?

This MR removes the specific rake setup from the diff-config command because we added a general set up in !3844 (merged).

If we set up twice, lib/tasks/config.rake is required twice, so task templates are added to the ConfigTasks instance twice, causing support/makefiles/Makefile.config.mk to have duplicate targets as reported in !3844 (comment 1997450885).

How to set up and validate locally

1. Run gdk diff-config
2. See that there is no diff (unlike on main right now)

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 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.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.