Thanks for your post! The Redmine defect plugin was built to work with a standard Redmine configuration, and it wouldn't be possible to customize the field configuration on the TestRail Cloud edition. This would only be possible on the TestRail Server edition, as you have the ability to customize the defect plugin and add/remove any field. If you're interested in learning more about this for the server edition you can have a look on our website here:
Hope this helps!