@Kampfarsch
I think you got the wrong story but I can see how you would counting a lot of them are similar.
This one is about the greatest assassin who beat the demon lord by himself, the demon lord on her part had a promise to fall in love with the person that beat her, when mc informed the king of his victory mc said give the credit to the hero group and only wanted a “normal” life.
He by no mean has a trash skill, mc is op and knows it but is trying his best to be normal (not that he knows what that means).
The trash skill one you might be thinking of is maybe “mapping” but then again there are a s**t ton of “trash” skill story’s so ehh