Remove canonical vertices #59

Canonical vertices were an experiment for the old CSG path: each vertex computed from the planes it lies on, so brushes sharing a corner get the same position. It never shipped (kCanonicalVertexStage is 0, off), and the exact CSG replaced the approach. With the old path gone (#58), what is left is dead code: CanonicalVertices, the stage and alignment switches, the neighbour snapping in the base polygon job that only the old path needed, and their tests.