Remove failing mails_from default
Remove failing mails_from default

As example.com is not a verified domain, Mandrill rejects the email
giving "reject_reason": "unsigned". Also, the mails_from setting is
already defined as a MailMethod preference.

oeoeaio/openfoodnetwork,openfoodfoundation/openfoodnetwork,Matt-Yorkley/openfoodnetwork,Matt-Yorkley/openfoodnetwork,openfoodfoundation/openfoodnetwork,lin-d-hop/openfoodnetwork,mkllnk/openfoodnetwork,oeoeaio/openfoodnetwork,mkllnk/openfoodnetwork,lin-d-hop/openfoodnetwork,lin-d-hop/openfoodnetwork,openfoodfoundation/openfoodnetwork,mkllnk/openfoodnetwork,Matt-Yorkley/openfoodnetwork,lin-d-hop/openfoodnetwork,oeoeaio/openfoodnetwork,openfoodfoundation/openfoodnetwork,mkllnk/openfoodnetwork,oeoeaio/openfoodnetwork,Matt-Yorkley/openfoodnetwork