Cleanup: Remove unused/unecessary OpenVDB C API

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 (see D12100).

Differential Revision: https://developer.blender.org/D12097

source/blender/blenkernel/BKE_mesh_remesh_voxel.h

@@ -23,10 +23,6 @@
  * \ingroup bke
  */
 
-#ifdef WITH_OPENVDB
-#  include "openvdb_capi.h"
-#endif
-
 #ifdef __cplusplus
 extern "C" {
 #endif