Fix create_unique_plan() so it doesn't generate useless entries in the

output targetlist of the Unique or HashAgg plan.  This code was OK when
written, but subsequent changes to use "physical tlists" where possible
had broken it: given an input subplan that has extra variables added to
avoid a projection step, it would copy those extra variables into the
upper tlist, which is pointless since a projection has to happen anyway.