Fix warning in OPDS importer settings

[jonathangreen]

Description

Move the _NO_DEFAULT_AUDIENCE to a private attribute.

Motivation and Context

After #1189 was merged, I am seeing this warning in the logs when starting the CM:

circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "core.opds_import.OPDSImporterSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.378728+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "api.opds_for_distributors.OPDSForDistributorsSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.378739+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "core.opds_import.OPDSImporterSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.378760+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "core.opds2_import.OPDS2ImporterSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.378831+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "api.opds_for_distributors.OPDSForDistributorsSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.379070+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "core.opds_import.OPDSImporterSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.379103+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "api.opds_for_distributors.OPDSForDistributorsSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.379128+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "core.opds2_import.OPDS2ImporterSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.379181+00:00"}
circulation-webapp-1  | {"host": "f0b88606839f", "app": "simplified", "name": "core.opds2_import.OPDS2ImporterSettings", "level": "WARNING", "filename": "settings.py", "message": "NO_DEFAULT_AUDIENCE was not initialized with FormField, skipping.", "timestamp": "2023-06-21T17:32:30.379295+00:00"}

Moving this to a private attribute means we won't see these warnings every time we start up the CM.

How Has This Been Tested?

Testing locally in docker container.

Checklist

• I have updated the documentation accordingly.
• All new and existing tests passed.

[RishiDiwanTT]

I knew about this warning, and intended to fix it during the PR cleanup, but that got out of hand 😆