Build: Remove manifest files; move metadata to source files

[scottgonzalez]

No description provided.

[tjvantoll]

For my own benefit what consumes this metadata? The release script? The changes themselves look fine.

[scottgonzalez]

label, group, and description are consumed by download builder. The rest of the metadata was only for the plugin registry, which is no longer being populated. I kept docs and demos because I think we may want to eventually use them in download builder to provide additional context for users.

[tjvantoll]

Ok that makes sense. And I like keeping docs and demos since we already have that information.

[scottgonzalez]

@rxaviers Can you just confirm that this looks good to land? We'll need to update download builder for 1.12, but this should get us one step closer to the merge.

[rxaviers]

I haven't tested it. But, it does look good if we are ok dropping the category descriptions.