Tet remeshing - remove assertion in Adaptive sizing field (CGAL#8378)

## Summary of Changes

The assertion is invalid (at least) when the function takes a far
vertex, inserted by parallel version of Mesh_3.

## Release Management

* Affected package(s): Mesh_3
* License and copyright ownership: unchanged

Tetrahedral_remeshing/include/CGAL/Adaptive_remeshing_sizing_field.h

@@ -497,7 +497,6 @@ average_edge_length_around(const Vertex_handle v, const Tr& tr,
  break;
  }
 
- CGAL_assertion(!edges.empty());
  if (edges.empty())
  return 0;