Blender Git Loki

Fix an own regression introduced in the dyntopo cleanup commits. Do not
remove a vertex from the pbvh if the only face it was part of was
removed from the pbvh. This should work but no time to investigate now
properly, and better have a solid release. The problem could be seen
when sculpting to open meshes. Some vertices and faces on the boundaries
would become stuck and unresponsive.