plugins: Remove legacy slideshow source

[cg2121]

Description

This removes the old slideshow source. Since the properties are the same for both of the new and old slideshows, they are backwards compatible.

Motivation and Context

It is just extra code that is not need.

How Has This Been Tested?

Created slideshow with old source and made sure it migrated to the new one.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• I have read the contributing document.
• My code has been run through clang-format.
• My code follows the project's style guidelines
• My code is not on the master branch.
• My code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.

[Warchamp7]

What happens to a Slideshow v1 and v2 in a scene collection after these changes?

I presume they're simply treated as the same thing which begs the question of why v1 was ever retained in the first place?

[cg2121]

I've added back the original slideshow_info struct, so users with old versions can seamlessly upgrade to the new source.

As an aside, we need to replace the versioning system, as it produces so much duplicate code.