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
