Remove metaprogramming-related keyword list koans

Too advanced at this point. Will considering including something like
this in #21.

lib/koans/06_keyword_lists.ex

@@ -37,26 +37,4 @@ defmodule KeywordLists do
 
     assert transform.("good", upcase: true) == ___
   end
-
-  def foo(kw_list), do: kw_list
-
-  koan "Actually function bodies are a sneaky use of keyword lists" do
-    list = foo do
-      :good
-    end
-
-    assert list == [do: ___]
-  end
-
-  def iff(kw_list), do: kw_list
-
-  koan "Turns out our beloved if statements are also using keyword lists" do
-    list = iff do
-      :this
-    else
-      :that
-    end
-
-    assert list == [do: ___, else: ___]
-  end
 end

test/koans/keyword_lists_koans_test.exs

@@ -10,8 +10,6 @@ defmodule KeywordListsTests do
       {:multiple, [:foo, "bar"]},
       "foo",
       "GOOD",
-      :good,
-      {:multiple, [:this, :that]},
     ]
 
     test_all(KeywordLists, answers)