pgsql: Remove unstable KNN SP-GiST test

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

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/340536dd489286d0f98878843390d92ddc593be0

Modified Files
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src/test/regress/expected/create_index_spgist.out | 10 ----------
src/test/regress/sql/create_index_spgist.sql      |  4 ----
2 files changed, 14 deletions(-)