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.

src/test/regress/expected/polymorphism.out

@@ -613,14 +613,3 @@ create aggregate build_group(int8, integer) (
   SFUNC = add_group,
   STYPE = int8[]
 );
--- check that we can apply functions taking ANYARRAY to pg_stats
-select distinct array_eq(histogram_bounds,histogram_bounds) from pg_stats
-where histogram_bounds is not null;
- array_eq 
-----------
- t
-(1 row)
-
--- such functions must protect themselves if varying element type isn't OK
-select max(histogram_bounds) from pg_stats;
-ERROR:  cannot compare arrays of different element types

src/test/regress/sql/polymorphism.sql

@@ -426,10 +426,3 @@ create aggregate build_group(int8, integer) (
   SFUNC = add_group,
   STYPE = int8[]
 );
-
--- check that we can apply functions taking ANYARRAY to pg_stats
-select distinct array_eq(histogram_bounds,histogram_bounds) from pg_stats
-where histogram_bounds is not null;
-
--- such functions must protect themselves if varying element type isn't OK
-select max(histogram_bounds) from pg_stats;