It is not possible to delete a field boundary once one has been created.
However, if you wish to disassociate a boundary from the current cropping record, you can change the region attached to the cropping record:
Open the cropping record for the field in question
Select the ‘Region’ subtab
Select ‘Swap Field Region’
Select ‘Setup Field Regions’
Select ‘Add Whole Field Region’ or ‘Add Part Field Region’ as appropriate. Rename the new or old regions as required, then ‘OK’ to close.