Allow RTE_SUBQUERY rels to be considered parallel-safe.

There isn't really any reason not to; the original comments here were
partly confused about subplans versus subquery-in-FROM, and partly
dependent on restrictions that no longer apply now that subqueries return
Paths not Plans.  Depending on what's inside the subquery, it might fail
to produce any parallel_safe Paths, but that's fine.

Tom Lane and Robert Haas