In the mobile app
Select a field from the list to open the field card. Pull it up, and you'll see the 'Edit or delete field' button at the bottom. Tap it to see the 'Delete from this season' option. All done!
In the web app
In the 'Fields' tab, select a field from the list, click the three dots next to it, and select 'Delete'.
Or, go to the field editing menu and select 'Remove field from ...'.
In the web application you can delete several fields or all fields of a season at once. To do this, click on the pencil above the list of all fields, select the fields to be deleted and click "Delete".
All the changes you save in the web app will be applied to your fields in the mobile app, too.