No longer exclude Overdrive audiobooks by default.

[tdilauro]

Description

Removes Overdrive from the (now empty) list of data sources whose audiobooks are excluded by default when no site-wide Excluded audiobook sources setting is present.

I will submit a corresponding core submodule commit on

Motivation and Context

The value of this setting has been reduced by new support for OverDrive audiobooks and its presence in the code -- in addition to the site-wide configuration option -- can cause confusion.

How Has This Been Tested?

• Manually tested to confirm that OD audiobook content appears in the client feed.
• Ran both the test suite for this repository and the one for the circulation manager repo.

Checklist:

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