Cleanup MailingAB BAO

Cleanup MailingAB BAO

- Use writeRecord function instead of boilerplate.
- Use api defaults for domain_id instead of setting in BAO:
  - APIv3 default domain is set via _civicrm_api3_mailing_a_b_create_spec() function.
  - APIv4 FieldDomainIdSpecProvider will set it automatically based on the required property in the schema.