BulletNifLoader: Replicate node bounds handling more closely

While there's no known apparent issues with the current behavior when most vanilla and modded assets are used, I knew for a while some things were off about it and it always seemed to be hanging on by a thread.
Collision mode flags and various bounding volumes are in fact unused in Morrowind. Morrowind looks for the node named "Bounding Box" (case-insensitive) to determine actor model bounds, which coincidentally might have the bounding box collision mode flag on and will be the node most likely to be used in OpenMW as well, but all collision mode flags are overwritten by the runtime. If it's not present, the bounds are autogenerated. Replicating all that allows to simplify some logic.
I don't seem run into anything wrong on vanilla, but this may need testing on a heavily modded setup, as this does change many assumptions about nodes with defined bounds. There should be no ill effect for actors, but there might be some gotcha about geometry collision (as is the case with the cryptic comment about a TR asset from 10 years ago, as it might have or have not targeted an asset issue).