Fix provided by Piotr Makal in patch D14204.
This patch fixes volume grid duplication which was occurring during
importing USD files. This was caused by calling BKE_volume_grid_add
twice per grid (excluding 'density' grid) for the same Volume
object: (1) in USDVolumeReader::read_object_data and (2) later in
BKE_volume_load.
Differential Revision: https://developer.blender.org/D14204
ef228d92f2