PO Number: Once a prefix or suffix is selected you cannot remove them

How to reproduce:

• Log into folio-testing
• Go to Orders=>Settings and make sure there are some prefixes and suffixes active
• Open Orders app
• Create new order
• Select a prefix

Expected: User is able to remove the prefix by selecting "none" from the dropdown list

Actual: User can NOT remove the prefix, they can only select a different one. User must start a new order to clear the prefix and suffix fields.