You can delete fields visible in the Member Database in 2 places. The first is directly in the Member Database:
Open the field you want to remove by clicking on the Field Name. Click Edit Field:
In the pop-up window, click Delete Field in the bottom left corner:
Click Yes to confirm that you want to delete this field:
To delete the field from the Registration Form, open the Registration form (under Group Settings):
Click on the field you want to delete and click on Delete Field in the bottom:
Confirm you want to delete the field by clicking Yes.
Your field is now deleted from this form.