Further fix pg_upgrade crossversion test for adminpack.

The DROP DATABASE step needs an "if exists" option, as the oldest
branches we test don't have the contrib_regression_adminpack DB.
Also remove unnecessary command to drop the extension from the
regression database; no version has installed it there during
buildfarm testing.

Discussion: https://postgr.es/m/0CFB76D0-0510-48B2-9916-1199F93BC28C@yesql.se

Author: tglsfdc

src/test/perl/PostgreSQL/Test/AdjustUpgrade.pm

@@ -109,10 +109,8 @@ sub adjust_database_contents
 	# we removed the adminpack extension in v17
 	if ($old_version < 17)
 	{
-		_add_st($result, 'regression',
-			'drop extension if exists adminpack');
 		_add_st($result, 'postgres',
-			'drop database contrib_regression_adminpack');
+			'drop database if exists contrib_regression_adminpack');
 		delete($dbnames{'contrib_regression_adminpack'});
 	}