Remove ExactSizeIterator::is_empty (See #41)

It seems we have no easy way to detect the use of the unstable feature
"exact_size_is_empty", and this function is not required since it defaults
to checking if `.len() == 0`.
Additionally, this function is not used, so removing it.

Author: chifflier

src/asn1_types/oid.rs

@@ -366,11 +366,6 @@ impl<'a, N: Repr> ExactSizeIterator for SubIdentifierIterator<'a, N> {
                 .count()
         }
     }
-
-    #[cfg(feature = "exact_size_is_empty")]
-    fn is_empty(&self) -> bool {
-        self.oid.asn1.is_empty()
-    }
 }
 
 impl<'a> fmt::Display for Oid<'a> {