Don't zero tuple_fraction when planning UNIONs with ORDER BYs

Don't zero tuple_fraction when planning UNIONs with ORDER BYs

Since 66c0185a3, the planner is able to use Merge Append -> Unique to
implement UNION queries and each subquery is prompted to produce Paths
correctly sorted by the UNION's targetlist.

Here we remove some now redundant code which was zeroing the
tuple_fraction at the parent level.  This will allow the planner to
consider cheap startup paths when planning the UNION's subqueries.

EXCEPT and INTERSECT set operations still have the tuple_fraction zeroed
in generate_nonunion_paths().  These operations currently always read
all of their subqueries' tuples.

Reported-by: Tom Lane
Discussion: https://postgr.es/m/3703023.1711654574@sss.pgh.pa.us