fix: remove system column "key" on filters

[zoidbergx]

Changes in this pull request

remove system column "key" on grid filters as the other system columns in grid.js
the system colums are defined in a constant in pimcore/bundles/AdminBundle/Controller/Admin/DataObject/DataObjectHelperController.php

const SYSTEM_COLUMNS = ['id', 'fullpath', 'key', 'published', 'creationDate', 'modificationDate', 'filename', 'classname'];

HOW

• open following object: https://demo.pimcore.com/admin/login/deeplink?object_81_object
• go to the Children Grid and add "key" from the system column vai Grid Options
  
• open browsers developer toolbar and apply the changes
• note the console logs
  

[github-actions]

Review Checklist

• Target branch (10.5 for bug fixes, others 11.x)
• Bug fix: check if files are affected that were moved to a bundle - create a PR there if applicable
• Tests (if it's testable code, there should be a test for it - get help)
• Docs (every functionality needs to be documented, see here)
• Migration incl. install.sql (e.g. if the database schema changes, ...)
• Upgrade notes (deprecations, important information, migration hints, ...)
• Label
• Milestone