Remove unstable KNN SP-GiST test

6cae9d2c10 introduced test for NULL values in KNN SP-GiST.  This test relies on
undetermined ordering showing different results on various platforms.  This
commit removes that test.  Will be replaced with better test later.

Discussion: https://postgr.es/m/6d51305e1159241cabee132f7efc7eff%40xs4all.nl
Backpatch-through: 12

src/test/regress/expected/create_index_spgist.out

@@ -555,16 +555,6 @@ WHERE seq.dist IS DISTINCT FROM idx.dist;
 ---+------+---+---+------+---
 (0 rows)
 
--- check ORDER BY distance to NULL
-SELECT (SELECT p FROM kd_point_tbl ORDER BY p <-> pt LIMIT 1)
-FROM (VALUES (point '1,2'), (NULL), ('1234,5678')) pts(pt);
-      p      
--------------
- (59,21)
- (9853,112)
- (1239,5647)
-(3 rows)
-
 EXPLAIN (COSTS OFF)
 SELECT count(*) FROM radix_text_tbl WHERE t = 'P0123456789abcdef';
                          QUERY PLAN                         

src/test/regress/sql/create_index_spgist.sql

@@ -225,10 +225,6 @@ SELECT * FROM quad_point_tbl_ord_seq3 seq FULL JOIN kd_point_tbl_ord_idx3 idx
 ON seq.n = idx.n
 WHERE seq.dist IS DISTINCT FROM idx.dist;
 
--- check ORDER BY distance to NULL
-SELECT (SELECT p FROM kd_point_tbl ORDER BY p <-> pt LIMIT 1)
-FROM (VALUES (point '1,2'), (NULL), ('1234,5678')) pts(pt);
-
 
 EXPLAIN (COSTS OFF)
 SELECT count(*) FROM radix_text_tbl WHERE t = 'P0123456789abcdef';