pgsql: Stabilize test of BRIN parallel create

Stabilize test of BRIN parallel create

As explained in 4d916dd876, the test instability is caused by delayed
cleanup of deleted rows. This commit removes the DELETE, stabilizing the
test without accidentally disabling parallel builds.

The intent of the delete however was to produce empty ranges, and test
that the parallel index build populates those correctly. But there's
another way to create empty ranges - partial indexes, which does not
rely on cleanup of deleted rows.

Idea to use partial indexes by Matthias van de Meent, patch by me.

Discussion: https://postgr.es/m/95d9cd43-5a92-407c-b7e4-54cd303630fe%40enterprisedb.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0bd4b0689ba1f12fbbd9919ca76a71df3e7702a2

Modified Files
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contrib/pageinspect/expected/brin.out | 13 ++++++-------
contrib/pageinspect/sql/brin.sql | 15 ++++++---------
2 files changed, 12 insertions(+), 16 deletions(-)