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,
Stabilize test of BRIN parallel create
The test for parallel create of BRIN indexes added by commit 8225c2fd40
happens to be unstable - a background transaction (e.g. auto-analyze)
may hold back global xmin for the initial VACUUM / CREATE INDEX. If the
cleanup happens before the next CREATE
