Boxer Tyson Fury -- Retires With Epic Tweet ... 'Go Suck a D***' (UPDATE)

8:28 AM PT -- Well, that was quick ... Fury now says the retirement post was just a joke -- and he DOES plan on fighting again.  "Hahahaha u think you will get rid of the GYPSYKING that easy," Fury said ... "I'm here to stay." He added, "Soon…