Remove json model mesh splitting to work around uniform limits

[mvaligursky]

Fixes #518

• meshes are no longer split into larger number of meshes using smaller number of bones
• these days we have pretty much guaranteed 256 vectors, which equates to around 60 bones - which is a reasonable number for low level devices. This path is also only used on WebGL1.
• this partitioning code was used only for json models, and not glb

[willeastcott]

I wrote this and I'm glad to see it go! 🚀