The poem "Death the Leveller" by James Shirley tells of the futility of the trappings of human pride in the face of death, because death comes to all. Shirley points out that genealogy, the country where someone is born, and victory and honors in battle will not save a person from death. He writes that heroes should not brag anymore about their "mighty deeds" and that only the "actions of the just smell sweet and blossom in the dust." In other words, only the actions of just people will be remembered and appreciated after death.
The phrases mentioned in the question symbolize the fact that death is impartial and it comes to all. A crown is what a king wears on his head, and a sceptre is a staff of authority that a king holds in his hand, so "sceptre and crown must tumble down" means that kings must die too, just like everybody else. Scythes and spades are implements that poor people use in agriculture, so "in the dust be equal made with the poor crooked scythe and spade" means that in death, kings are no different than poor people. Once they are dead, they are all the same, unless they were just people who performed actions worthy of remembrance.