Before we explain how you can delete a field from your pipe's phases or form, we need to warn you that, when you delete a field, all the information you added to that field will be gone as well, so be very careful before deleting any field!
Deleting a field from a phase/form is really easy. If you wish to delete a field from your pipe's form, you'll open the form. If it's a field from a phase, you'll open a card in the phase you want to delete the field in.
Then, click on the three dots next to the field's name and on delete field. After you confirm, your field will be deleted.