This commit uses OpenVDB more directly for the voxel remesher, without
the extra indirection of copying to a Blender API. This makes the code
simpler, shorter, and easier to understand (though I didn't observe any
performance improvement).

This also removes the rest of the unused and undocumented OpenVDB C API ,
which was written when Blender's code didn't really use C++, and doesn't serve
a purpose anymore. Those features will be implemented as nodes in the
future anyway (hopefully soon).