Can I withdraw from the poster session? I submitted my paper to ICANN2016 conference in which we could choose among the possible options of "poster" and "oral" presentation at the submission time. I submitted for the oral category and received the acceptance notice as a "full paper" to be in the proceedings, but no info on the type of presentation (oral/poster).

Then I registered as they requested to do so before the deadline, but later on, when the conference program become revealed on the website I noticed that I am placed in one of the poster sessions and even in a so "irrelevant" session! 

Honestly I become less motivated to participate in the conference as I've heard that the posters are mostly a way the organizers try to compensate the conference expenses (they have 80 posters in the conference). And the chances of getting noticed or getting any useful feedback on my work can be so subtle!
Also not to mention that in the review feedback there was no detail about the review process which could be considered as useful feedback on my work. Just a simple general comment like "the work is so solid, technical and great.."

So I came to a sort of conclusion that I already wasted my publication by submitting it to the conference!

Also I was wondering if I can withdraw from participation and ask my payment back? considering that they didn't mention the type of the presentation in the acceptance notice, and also the fact that I chose the oral presentation in the submission.
This is very, very common. At most conferences, there are simply not enough time slots for everyone to give an oral presentation. Conferences still tend to be way too long most of the time with way too many "less important" presentations between "interesting" ones anyway. 
Posters can be a very efficient way of communicating your results and connecting with other researchers important to you. You are not "wasting" your paper by presenting a poster. And you will attend other's presentations and can engage in the discussions. So I don't really agree with your conclusion.

It's understandable that you are disappointed. However, while it is totally up to you, your decision on whether to participate should not depend on a potential resentment of being "rejected" from an oral presentation. Base your decision on whether to go on who you think you might get to know, and who might be there who could benefit from your results. Prepare a great poster. Have handouts ready. Approach people directly, start to network actively. 

Whether or not you might receive any money back highly depends on the specific event and the local organizing committee. You can ask. But at this stage I would say it's unlikely.