Johnsson had been with City for a year but requested to leave because he wanted to move back home and give up professional football.
The 27-year-old midfielder will now return to B36-Thorshavn and play amateur football. It is also expected that he will end his international career although that has not been confirmed.
Meanwhile, 26-year-old winger David Beresford has completed his move to Plymouth, signing a three-year deal for the Division Two side.