Remove the "does not import" warning

There's no reason to show this warning:

Technically this addon is in the "Import-Export" category, which is one of the pre-defined categories found in when you open Blender's user settings.

However, showing a useless warning next to your add-on only [confuses users](https://blender.stackexchange.com/questions/42834/blender-add-on-import-export-three-js-doesnt-activate)