To remove a Field from a Card, you'll need to remove it from the Card Category. When you remove a Field from a Card Category, you are not deleting the Field from the Project, but just from the Category and any existing Cards and new Cards created for this Category.
Note, any data entered for this Field will be lost from existing Cards of this Category.
Remove a Field:
1. Click your avatar on the bottom-left to open the User menu.
2. Select "Project Settings".
3. Click on the "Categories" tab. All the Card Categories for your Project will be listed.
4. Find the Category you want to remove the Field from, and double click in the Fields' cell to open the "Edit Fields in Card Category window". The top window shows all Fields for this Category.
5. Find the Field you want to remove and click the bin icon next to it. Then click "Confirm".
The Field will be removed from all existing Cards and new Cards created for this Category.
For more help on fields see the Fields Help Guide.