Remove the klist_t(poly) backed bsp_subdivide()

This removes the last caller of `_bsp_subdivide()` and then removes it entirely.

This also early-terminates clipping much earlier when there is nothing left to clip.

Merge target: #13 

Timing now

```
real	0m21.014s
user	0m20.906s
sys	0m0.093s
```