Eliminates Iterable

Which at least makes the example compile. Iterable is not used in the
other examples, so I don't really get what it's doing there. It's not
really relevant to the main topic, which is subscripts. The example
should probably be improved, but that's a different thing
altogether. Closes #1438.

Author: JJ

doc/Language/subscripts.pod6

@@ -672,7 +672,7 @@ to that attribute (using a custom type constraint to make sure users don't
 insert anything invalid into it):
 
     =begin code :skip-test
-    class HTTP::Header does Associative does Iterable {
+    class HTTP::Header does Associative {
         subset StrOrArrayOfStr where Str | ( Array & {.all ~~ Str} );
 
         has %!fields of StrOrArrayOfStr