AttenuationTest is incorrectly tagged with core

[Andreas Plesch]

The AttenuationTest metadata includes the #core tag and is listed with

https://github.com/KhronosGroup/glTF-Sample-Assets/blob/main/Models/Models-core.md

However, its description starts with This model tests interactions between attenuation color, attenuation distance, and thickness in the KHR_materials_volume extension.

The #core tag should be removed.

[Ed Mackey]

PR merged. This should be fixed the next time the readmes are regenerated. Thanks @andreasplesch.

[Andreas Plesch]

Thanks.