Reinstate dropping schema for gporca test suite

Partition tables hard-code the operator '=' lookup to namespace
'pg_catalog', which means that in this test we had to put our
user-defined operator into that special system namespace. This works
fine, until we try to pg_dump the resulting database: pg_catalog is not
dumped by default. That led to an incomplete dump that will fail to
restore.

This commit reinstates the dropping of the schema at the end of `gporca`
test to get the pipelines back to green (broken after c7ab6924 ).
Backpatch to 5X_STABLE.

src/test/regress/sql/gporca.sql

@@ -1435,3 +1435,7 @@ drop table bar;
 create table bar(name text);
 insert into bar values('person');
 select * from unnest((select string_to_array(name, ',') from bar)) as a;
+
+-- start_ignore
+DROP SCHEMA orca CASCADE;
+-- end_ignore