pgsql: Remove newly-added regression test cases that referenced

Log Message:
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Remove newly-added regression test cases that referenced pg_stats.
The proposed fix for this is a behavioral change that probably shouldn't
get back-patched, and it doesn't seem worth putting a workaround into
a back branch.

Tags:
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REL8_3_STABLE

Modified Files:
--------------
pgsql/src/test/regress/sql:
polymorphism.sql (r1.5.2.1 -> r1.5.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/sql/polymorphism.sql?r1=1.5.2.1&r2=1.5.2.2)
pgsql/src/test/regress/expected:
polymorphism.out (r1.12.2.1 -> r1.12.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/test/regress/expected/polymorphism.out?r1=1.12.2.1&r2=1.12.2.2)