If that's a ground wire from the battery to the coil, you don't need it. First, the coil's body does not need a ground, and second, if you have it attached to the negative post on the coil, it won't spark.
The coil negative should only have the wire from the points to it. Sometimes a tach will hook there too, but if not running a tach, the distributor wire is the only one.
And just in case you're starting from scratch, the largest battery cable from the negative side goes to the engine block, while another smaller jumper should go to the body. And even one to the frame if you have that option. If not, jumpering ground wires/cables/straps between the engine and body, engine and frame, and anywhere else you care to put them, can make for a much better performing electrical system.