src/Cabal2Nix/Flags.hs: remove obsolete configuration for pandoc
src/Cabal2Nix/Flags.hs: remove obsolete configuration for pandoc

bennofs/cabal2nix,gridaphobe/cabal2nix,psibi/cabal2nix,dmalikov/cabal2nix,jb55/cabal2nix,Fuuzetsu/cabal2nix,spencerjanssen/cabal2nix