Use DELETE instead of UPDATE to speed up vacuum test

e2e992820f introduced a test which generated dead tuples for
vacuum with an UPDATE. The test only required enough dead TIDs for two
rounds of index vacuuming. This can be accomplished with a DELETE
instead of an UPDATE -- which generates about 50% less WAL and makes the
test 20% faster in many cases. The test takes several seconds (more on
slow buildfarm animals) because we need quite a few tuples to trigger
two rounds of index vacuuming; so it is worth a follow-on commit to
speed it up.

Suggested-by: Masahiko Sawada
Discussion: https://postgr.es/m/CAAKRu_bWmMjmqL%2BOZ2duEQ80u7cRvpsExLNZNjzk-pXX5skwMQ%40mail.gmail.com
Backpatch-through: 14, the first version containing this test.

src/test/recovery/t/043_vacuum_horizon_floor.pl

@@ -70,7 +70,7 @@ $node_primary->safe_psql($test_db, qq[
 	CREATE TABLE ${table1}(col1 int) with (autovacuum_enabled=false);
 	INSERT INTO $table1 SELECT generate_series(1, 200000);
 	CREATE INDEX on ${table1}(col1);
-	UPDATE $table1 SET col1 = 0 WHERE col1 > 1;
+	DELETE FROM $table1 WHERE col1 > 1;
 	INSERT INTO $table1 VALUES(1);
 ]);