Migrate unused models and units to artwork repo

[GoogleFrog]

The majority of the download size of ZK itself is in the textures for the models. If a model is not currently being used it may as well be moved to artwork. This is not vital but it is worth remembering.

[sprunk]

Unused models are modder fodder so maybe it's a good idea to keep them around after all as a general rule. Removal would have probably made the most sense around the steam release.