Remove unused variables

Author: WardBrian

stan/math/prim/constraint/sum_to_zero_constrain.hpp

@@ -75,7 +75,6 @@ template <typename Mat, require_eigen_matrix_dynamic_t<Mat>* = nullptr,
 inline plain_type_t<Mat> sum_to_zero_constrain(const Mat& x) {
   const auto N = x.rows();
   const auto M = x.cols();
-  const auto s = std::max(N, M);
 
   plain_type_t<Mat> Z = Eigen::MatrixXd::Zero(N + 1, M + 1);
   if (unlikely(N == 0 || M == 0)) {

stan/math/prim/constraint/sum_to_zero_free.hpp

@@ -77,7 +77,6 @@ inline plain_type_t<Mat> sum_to_zero_free(const Mat& z) {
 
   const auto N = z_ref.rows() - 1;
   const auto M = z_ref.cols() - 1;
-  const auto s = std::max(N, M);
 
   plain_type_t<Mat> x = Eigen::MatrixXd::Zero(N, M);
   if (unlikely(N == 0 || M == 0)) {

stan/math/rev/constraint/sum_to_zero_constrain.hpp

@@ -93,9 +93,6 @@ inline auto sum_to_zero_constrain(T&& x) {
   reverse_pass_callback([arena_x, arena_z]() mutable {
     const auto Nf = arena_x.val().rows();
     const auto Mf = arena_x.val().cols();
-    const auto N = Nf + 1;
-    const auto M = Mf + 1;
-    const auto s = std::max(Nf, Mf);
 
     Eigen::VectorXd d_beta = Eigen::VectorXd::Zero(Nf);