Remove some redundant set_cheapest() calls

I happened to notice that the set_cheapest() calls in functions
set_namedtuplestore_pathlist() and set_result_pathlist() are redundant,
as we've centralized the set_cheapest() calls in set_rel_pathlist().

Attached is a trivial patch to remove these calls.

BTW, I suspect that the set_cheapest() call in set_dummy_rel_pathlist()
is also redundant. The comment there says "This is redundant when we're
called from set_rel_size(), but not when called from elsewhere". I
doubt it. The other places where it is called are set_append_rel_size()
and set_subquery_pathlist(), both being called from set_rel_size(). So
set_cheapest() would ultimately be called in set_rel_pathlist().

Thoughts?

Thanks
Richard