This is a horse term. Flogging is beating with a whip. It does no good to beat a horse that is dead, because it will never get back up and work for you.

beat the story mode

It's a useless action. (A dead horse won't pull your wagon no matter how much you beat him.)

"To flog a dead horse" is the English equivalent to "to beat a dead horse", which means to do something over and over again when it is not necessary, whether it be saying something or doing something.

It means that you should stop talking about the subject because it's useless and already been talked about enough. (A dead horse won't do anything for you, no matter how many times you beat it.)