message: refactor(header): remove unused VecMap.is_empty
diff: @@ -54,9 +54,6 @@ impl<K: PartialEq, V> VecMap<K, V> {
  #[inline]
  pub fn len(&self) -> usize { self.vec.len() }
  - #[inline]
  - pub fn is_empty(&self) -> bool { self.vec.is_empty() }
  -
  #[inline]
  pub fn iter(&self) -> ::std::slice::Iter<(K, V)> {
  self.vec.iter()
  