Here is how you can remove/delete a field from a data source:

1. Open the LAYER ADMIN panel. 
   
2. Click the Data Sources tab.
   
3. Select the data source you wish to modify.
   
4. In the "Fields" section of the details panel, click "REMOVE" (Remove an existing field).
   
5. Select the field you would like to remove.
   
6. Click "REMOVE FIELDS".