Retire Walters Art Museum provider script

[AetherUnbound]

Fixes

Fixes WordPress/openverse#1415 by @AetherUnbound

Description

This PR retires the Walters Art Museum DAG and removes its tests. I've also updated the DAGs.md file.

Testing Instructions

1. just test
2. just recreate -> verify that walters_workflow is not present in the DAG list

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main) or a parent feature branch.
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no visible errors.

Developer Certificate of Origin

Developer Certificate of Origin

Developer Certificate of Origin
Version 1.1

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Developer's Certificate of Origin 1.1

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    have the right to submit it under the open source license
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(b) The contribution is based upon previous work that, to the best
    of my knowledge, is covered under an appropriate open source
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[krysal]

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[zackkrida]

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