Who says, "let the dead bury the dead"? What does he mean?

Heck Tate says, “I’m not a very good man, sir, but I am sheriff of Maycomb County. Lived in this town all my life an‘ I’m goin’ on forty-three years old. Know everything that’s happened here since before I was born. There’s a black boy dead for no reason,
and the man responsible for it’s dead. Let the dead bury the dead this time, Mr. Finch. Let the dead bury the dead.”

In this quote, Heck is encouraging Atticus to let Ewell's death go.... to pursue his death would create the loss of one more innocent. Ewell, in essence, was responsible for Tom's death.... the death of an innocent man. To pursue the issue and allow Boo, who wanted nothing more than to protect Atticus' children, would result in yet another loss. In Heck's eyes, the dead, should simply bury the dead.