removed unused variable in Aiger backend.
removed unused variable in Aiger backend.

hoangmle/metaSMT,finnhaedicke/metaSMT,finnhaedicke/metaSMT,agra-uni-bremen/metaSMT,agra-uni-bremen/metaSMT,nbruns1/metaSMT,hoangmle/metaSMT,nbruns1/metaSMT,hoangmle/metaSMT,agra-uni-bremen/metaSMT,finnhaedicke/metaSMT,nbruns1/metaSMT