"Spade away" sounds close to "fade away". To fade away means to gradually grow faint and disappear. A spade is tool with sharp edges and a long handle that is used to dig up dirt. The job of a gravedigger is to dig graves, which means to dig up dirt in order to create that grave. The tool used to dig a grave can be a spade. The joke is playing on a gravedigger's "spade" which they used to replace the word "fade" so that "fade away" turned into "spade away". 
OLD GRAVEDIGGERS ; never die , they just spade away .