refactor(testimonials): remove case study column from admin list (#228)

## Description

Removed the `_caseStudy` column from the testimonials admin list view in
the ApostropheCMS admin interface.

## Changes

- Removed the `_caseStudy` column definition from `columns.add` in
`testimonials/index.js`
- The column previously displayed "Related Case Study" in the
testimonials list view

## Notes

- The `_caseStudy` field definition remains intact and functional
- The field filter for case studies remains available
- Frontend templates continue to display case study information
correctly
- This change only affects the admin UI list view display

## Impact

No breaking changes. This is a UI-only change that simplifies the admin
list view by removing a column that was not essential for quick
reference.

Author: IhorMasechko

website/modules/testimonials/index.js

@@ -106,10 +106,6 @@ module.exports = {
         label: 'Organization',
         name: 'organization',
       },
-      _caseStudy: {
-        label: 'Related Case Study',
-        name: '_caseStudy.0.title',
-      },
     },
   },
   filters: {