Do not use the inertia box for volume computation.

PiperOrigin-RevId: 582319067
Change-Id: I347d820f48faab15b609bcfc8295b07bd2d0f7ed

src/user/user_objects.cc

@@ -1332,12 +1332,7 @@ double mjCGeom::GetVolume(void) {
     }
 
     mjCMesh* pmesh = model->meshes[meshid];
-    if (model->exactmeshinertia || typeinertia==mjSHELL_MESH) {
-      return pmesh->GetVolumeRef(typeinertia);
-    } else {
-      const double* boxsz_volume = pmesh->boxsz_volume();
-      return boxsz_volume[0]*boxsz_volume[1]*boxsz_volume[2]*8;
-    }
+    return pmesh->GetVolumeRef(typeinertia);
   }
 
   // compute from geom shape

src/user/user_objects.h

@@ -615,7 +615,6 @@ class mjCMesh: public mjCBase {
   const std::vector<int>& userface() const { return userface_; }
 
   // mesh properites computed by Compile
-  const double* boxsz_volume() const { return boxsz_volume_; }
   const double* aamm() const { return aamm_; }
 
   // number of vertices, normals, texture coordinates, and faces