Fix self-mailer integration test: remove hardcoded billing_group_id

bg_5c79d158d8f69e3e0 is a postpaid billing group (is_for_lob_credits=false)
but the CI test account is on Lob Credits. The API enforces payment type
parity and throws billing_group_payment_type_mismatch when they differ.
Omitting billing_group_id lets the API auto-select the correct default
group for the account's payment type.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: kevinpjones

test/Integration/test_self_mailers_api.py

@@ -101,7 +101,6 @@ def setUpClass(self):
             merge_variables=MergeVariables(),
             inside = "https://s3.us-west-2.amazonaws.com/public.lob.com/assets/templates/self_mailers/6x18_sfm_inside.pdf",
             outside = "https://s3.us-west-2.amazonaws.com/public.lob.com/assets/templates/self_mailers/6x18_sfm_inside.pdf",
-            billing_group_id = "bg_5c79d158d8f69e3e0",
             use_type= SfmUseType("marketing")
         )