So how someone can remove or delete a topic that he started if he thinks it's irrelevant or unnecessary right now?
You can't. It's not up to you to decide your topic is irrelevant to someone else, that would for instance make it easier for scammers to cover their tracks.
I think there must be some procedure to do that because there are many topics in the service and marketplace board which is created for short time. Or just leave it by thinking it's not there.
If you're a honest trader, keeping records of past trades is actually beneficial for your reputation. If a thread served it's purpose (because you found what you're looking for): just lock it (bottom-left). If you think it not worth keeping on that board, move (bottom-left) it to
Archival.
If you initially create a self-moderated topic, then you can delete it later.
Nope, you can't.
I just tested it.