Revert "[FIX] fields: `digits()` computation"

This reverts commit eeedd2d9.

This revision introduces an issue more serious than the ones
it fixes. This is no longer possible to receive an email
aimed a sale.order thread with catchall.

To reproduce the issue:
 - Create a new sale order
 - Send a message in the thread to the customer
 - Reply to the mail received in the customer mailbox
 - Traceback, AttributeError: digits

 opw-640370