Simplify and merge unwanted-module drop logic in AdjustUpgrade.pm.

In be7800674 and followups, we failed to notice that there was
already a better way to do it: instead of using DROP DATABASE
IF EXISTS, we can check the list of existing DBs.  Also, there
seems no reason not to merge this into the pre-existing code
for getting rid of unwanted module databases.

Discussion: https://postgr.es/m/1066872.1710006597@sss.pgh.pa.us

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

@@ -86,7 +86,7 @@ sub adjust_database_contents
 
 	# remove dbs of modules known to cause pg_upgrade to fail
 	# anything not builtin and incompatible should clean up its own db
-	foreach my $bad_module ('test_ddl_deparse', 'tsearch2')
+	foreach my $bad_module ('adminpack', 'test_ddl_deparse', 'tsearch2')
 	{
 		if ($dbnames{"contrib_regression_$bad_module"})
 		{
@@ -94,6 +94,12 @@ sub adjust_database_contents
 				"drop database contrib_regression_$bad_module");
 			delete($dbnames{"contrib_regression_$bad_module"});
 		}
+		if ($dbnames{"regression_$bad_module"})
+		{
+			_add_st($result, 'postgres',
+				"drop database regression_$bad_module");
+			delete($dbnames{"regression_$bad_module"});
+		}
 	}
 
 	# avoid no-path-to-downgrade-extension-version issues
@@ -106,17 +112,6 @@ sub adjust_database_contents
 			'drop extension if exists test_ext7');
 	}
 
-	# we removed the adminpack extension in v17
-	if ($old_version < 17)
-	{
-		_add_st($result, 'postgres',
-			'drop database if exists contrib_regression_adminpack');
-		_add_st($result, 'postgres',
-			'drop database if exists regression_adminpack');
-		delete($dbnames{'contrib_regression_adminpack'});
-		delete($dbnames{'regression_adminpack'});
-	}
-
 	# we removed this test-support function in v17
 	if ($old_version >= 15 && $old_version < 17)
 	{