remove elementary MapSet.size/1 koan

size is covered under koan "However, I do not allow duplication"

lib/koans/09_map_sets.ex

@@ -9,10 +9,6 @@ defmodule MapSets do
     assert Enum.fetch(@set, 0) == {:ok, ___}
   end
 
-  koan "How large is my map set?" do
-    assert MapSet.size(@set) == ___
-  end
-
   koan "However, I do not allow duplication" do
     new_set = MapSet.new([1, 1, 2, 3, 3, 3])