Remove dead code

Eigen 5's simplified constructors now help the compiler flag "unused
variable" errors on dead matrices.

Author: jwnimmer-tri

examples/multibody/cart_pole/test/cart_pole_test.cc

@@ -143,7 +143,6 @@ TEST_F(CartPoleTest, SystemDynamics) {
   const Vector2<double> q(2.5, M_PI / 3);
   const Vector2<double> v(-1.5, 0.5);
 
-  Matrix2<double> M;
   cart_slider_->set_translation(context_.get(), q(0));
   cart_slider_->set_translation_rate(context_.get(), v(0));
   pole_pin_->set_angle(context_.get(), q(1));

math/test/rigid_transform_test.cc

@@ -854,7 +854,6 @@ GTEST_TEST(RigidTransform, SpecializedTransformOperators) {
 
   // Test ApplyAxialRotation().
   const Vector3d p_C(1.0, 2.0, 3.0);
-  Vector3d p_B;  // reusable result
   EXPECT_TRUE(CompareMatrices(Xform::ApplyAxialRotation<0>(xrX_BC, p_C),
                               xrX_BC * p_C, kTol));
   EXPECT_TRUE(CompareMatrices(Xform::ApplyAxialRotation<1>(yrX_BC, p_C),

math/test/rotation_matrix_test.cc

@@ -720,7 +720,6 @@ GTEST_TEST(RotationMatrix, AxialRotationOperators) {
 
   // Test ApplyAxialRotation().
   const Vector3d v_C(1.0, 2.0, 3.0);
-  Vector3d v_B;  // reusable result
   EXPECT_TRUE(CompareMatrices(Rmat::ApplyAxialRotation<0>(xR_BC, v_C),
                               xR_BC * v_C, kTol));
   EXPECT_TRUE(CompareMatrices(Rmat::ApplyAxialRotation<1>(yR_BC, v_C),
@@ -803,7 +802,6 @@ GTEST_TEST(RotationMatrix, AxialRotationOptimizations) {
 
     // But ... ApplyAxialRotation() should work anyway with this junk in place.
     const Vector3d v_C(1.0, 2.0, 3.0);
-    Vector3d v_B;
     EXPECT_TRUE(CompareMatrices(Rmat::ApplyAxialRotation<1>(yR_BC_mangled, v_C),
                                 yR_BC * v_C, kTol));
 

solvers/mixed_integer_rotation_constraint.cc

@@ -520,9 +520,6 @@ void AddBoxSphereIntersectionConstraints(
             double cos_theta = d;
             const double theta = std::acos(cos_theta);
 
-            Eigen::Matrix<double, 1, 6> a;
-            Eigen::Matrix<double, 3, 9> A_cross;
-
             Eigen::Vector3d orthant_normal;
             Vector3<Expression> orthant_c;
             for (int o = 0; o < 8; o++) {  // iterate over orthants

solvers/test/mathematical_program_test.cc

@@ -4369,9 +4369,6 @@ GTEST_TEST(TestMathematicalProgram, AddConstraintMatrix2) {
   ASSERT_EQ(prog.GetAllConstraints().size(), 1);
   ASSERT_EQ(prog.GetAllLinearConstraints().size(), 1);
 
-  Eigen::Matrix<double, 4, 2> A_expected;
-  Eigen::Matrix<double, 4, 1> lower_bound_expected;
-  Eigen::Matrix<double, 4, 1> upper_bound_expected;
   std::array<std::array<Eigen::RowVector2d, 2>, 2> coeff;
   coeff[0][0] << 1, 0;
   coeff[0][1] << 1, 2;