Adding back filter pmpro_deactivate_old_levels

[dparker1005]

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• Does your code follow the WordPress' coding standards?
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Changes proposed in this Pull Request:

Adding the pmpro_deactivate_old_levels filter back, which was lost in the 3.0 migration.

Resolves XXX.

How to test the changes in this Pull Request:

1. Install PMPro PBC
2. Set up a "donations only" level
3. See that purchasing that level removes previous membership before PR, but doesn't after PR.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you successfully run tests with your changes locally?

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.