A convicted murderer had a three-year prison sentence for failure to register as a sex offender reversed after a state appeals panel found Cook County prosecutors failed to prove why he was obligated to register 33 years after his initial conviction.The 1st District Appellate Court entered an unpublished order Monday detailing how the state could have proven that Kevin J. Jones was required to register weekly with his local police department — but it failed to do so.Jones was convicted of attempted rape on Sept. 4 …