Remove unused USE_NEW_CSS_PIPELINE envvar

What does this merge request do and why?

The new CSS pipeline is now the default and the USE_NEW_CSS_PIPELINE envvar was removed from the canonical repository in gitlab!145930 (merged).

This commit basically reverts the code added in !3537 (merged).

How to set up and validate locally

1. Run gdk reconfigure
2. Run gdk stop
3. Run gdk start

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.