[mlir][Quasipolynomials] Fix -Wunused-variable in QuasiPolynomial.cpp (NFC)

llvm-project/mlir/lib/Analysis/Presburger/QuasiPolynomial.cpp:29:39:
 error: unused variable 'aff' [-Werror,-Wunused-variable]
    for (const SmallVector<Fraction> &aff : term) {
                                      ^
1 error generated.