db: maintain 9.6 compatibility

I know it's generally frowned upon to make edits to existing migrations,
but this should be a transparent change that makes absolutely no
difference to existing users with the migration already applied.
However, PostgreSQL 9.6 users (still the default on Debian Stretch)
stand to gain a lot from this simple change.

db/20190913175445-delete-comments.sql

@@ -1,6 +1,6 @@
 DROP TRIGGER IF EXISTS commentsDeleteTrigger ON comments;
 
-DROP FUNCTION IF EXISTS commentsDeleteTriggerFunction;
+DROP FUNCTION IF EXISTS commentsDeleteTriggerFunction();
 
 ALTER TABLE comments
   ADD deleted BOOLEAN NOT NULL DEFAULT false;