Remove a piece of dead code.

Remove a piece of dead code which made select_range() and
select_reverse_range() extremely inefficient (a new iterator
instance for each tuple) and confusing.

box.index.iterator() creates a closure which is sufficient
to produce all iteration results. Use of index:next()
was apparently left by some tricky merge.

src/box/lua/box.lua

@@ -151,7 +151,6 @@ function box.bless_space(space)
                 break
             end
             table.insert(range, v)
-            iterator_state, v = index:next(iterator_state)
         end
         return unpack(range)
     end
@@ -162,7 +161,6 @@ function box.bless_space(space)
                 break
             end
             table.insert(range, v)
-            iterator_state, v = index:prev(iterator_state)
         end
         return unpack(range)
     end