Remove converter scripts.

[Mugen87]

Related issue: https://discourse.threejs.org/t/how-to-use-fbx2three-js/21870

Description

I'd like to suggest to remove fbx2three.js and obj2three.js and recommend different workflows instead:

• Use the editor for converting assets into glTF or JSON
• Or use existing command lines tools like FBX2glTF or obj2gltf to force a glTF based workflow. Especially FBX2glTF should provide better results than the current fbx2three.js script.

[mrdoob]

Sounds good! 👍

[mrdoob]

Thanks!