LifterLMS streamline mode and the /membership/ CPT slug

[MaximilianoRicoTabo]

• Unregister the CPT that resevers the 'memberships' slug.

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• Have you followed the Contributing guideline?
• Does your code follow the WordPress' coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Unregister llms_membership if streamline version of lifterlms is enabled.

Resolves #2820

How to test the changes in this Pull Request:

1. Create a page with 'memberships' slug
2. Check that it doesn't render appropriately
3. Apply this patch.
4. Flush cache saving permalinks.
5. Check again.

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.

[ideadude]

Instead of hooking into register_post_types and maybe adding the filter, we could just hook into register_post_type_args and put the streamline check into THAT callback function.

[MaximilianoRicoTabo]

Added suggested change. Tested at local, works perfect. Thanks @ideadude