Part of #118145.
Doing so removes the last usages of some dependent code as well, namely
the `SCULPT_VERTEX_NEIGHBORS_ITER` macro and the `PBVHVertRef` version
of the `flood_fill` algorithms. The now dead & unused code, as well as
their related helper functions, structs, and defines have been removed.
Additionally, a helper `vert_face_set_get` method for multires has been
added to be consistent with the other provided APIs
Pull Request: https://projects.blender.org/blender/blender/pulls/127781
b5d418a83f