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