From the last scene of Laugh God

Kidd's heart nearly stopped before he was pulling his partner in tight; a hug that would crush a lesser man.

Kidd would never bend a knee. But. "Thank you," his words lost into Killer's hair. But Luffy heard, "Monkey D. Luffy.... Pirate King."

Where Luffy's reality bending powers unburdened Killer of his unwanted Devil Fruit. Kidd relinquished any claim to the Pirate King, having his partner returned to himself more important than continuing to challenge for the title