Faber says, "Remember, the firemen are rarely necessary. The public itself stopped reading of its own accord." What does he mean?

Faber means that the job of the firemen, to destroy books when they are found, is becoming less and less necessary because not many people read books anymore.  The society in the story has become so busy in other, less educational pursuits, that even if there were books around, they probably wouldn't read.  Therefore, there aren't many books that need to be destroyed as fewer and fewer people bother to read.  This was one of the strong messages Bradbury was trying to send with the story; that if people become obsessed with self-indulgent activities, become concerned with political correctness to the point of blandness, become more concerned with entertainment than with enlightenment, then books will naturally become obsolete. He was warning the reader to avoid allowing this to happen.