LOS ANGELES — All good things must come to an end. It seems that goes for the “Worst,” also. The fifth and final season of the FXX series “You’re the Worst” will begin airing Jan. 9, bringing the dark and occasionally twisted take on romantic comedy to its conclusion. The cable series has examined love and happiness told through Gretchen Cutler (Aya Cash) and Jimmy Shive-Overly (Chris Geere), two people who prove you don’t have to be a good person to be a great partner.