[iter] Remove hb_len()

Not planning on using it.  So remove.  Can add later if needed.

src/hb-iter.hh

@@ -145,14 +145,6 @@ static const struct
 
 } hb_iter HB_UNUSED;
 
-static const struct
-{
-  template <typename T>
-  unsigned
-  operator () (T&& c) const
-  { return hb_iter (c).len (); }
-} hb_len HB_UNUSED;
-
 
 /* Mixin to fill in what the subclass doesn't provide. */
 template <typename iter_t, typename item_t = typename iter_t::__item_t__>

src/test-iter.cc

@@ -158,8 +158,6 @@ main (int argc, char **argv)
   | hb_drain
   ;
 
-  hb_len (src);
-
   t << 1;
   long vl;
   s >> vl;