Drop RE2 diagnostic

What does this merge request do and why?

Since re2 v2.0 (https://ruby.social/@mudge/111059216711881571), the gem now ships with precompiled native gems shipped with all batteries included so macOS and Linux users no longer need the libre2 system library to be installed. A a result, the diagnostic no longer is needed and can lead to false positives when bundle install is needed in the GitLab Rails directory.

How to set up and validate locally

Run gdk doctor with a gem missing in gitlab/. re2 should no longer be mentioned.

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.