The fact that you used to be on JSA is relevant information regarding this situation, as it explains why you have been referred to the Work Programme.
The requirement for you to attend ceased when your JSA claim ended.
Whilst you probably can ignore the letter, I would recommend that you contact the provider and make them aware that you are no longer required to participate, this should avoid their contacting the DWP about your failure to attend, and any confusion that that might ensue.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems