message: indexgenerator: Remove unused function
  VertexHasher::empty() isn't used; we pass empty key value to the
  hash lookup function directly.
diff: @@ -35,11 +35,6 @@ struct VertexHasher
  const char* vertices;
  size_t vertex_size;
  - unsigned int empty() const
  - {
  - return ~0u;
  - }
  -
  size_t operator()(unsigned int index) const
  {
  return murmurHash(vertices + index * vertex_size, vertex_size, 0);
  