Remove an unused variable from CudaSparseMatrix

Change-Id: Id9c969fda8d636dc793b98b452b2aaffacb11180

internal/ceres/cuda_sparse_matrix.cc

diff --git a/internal/ceres/cuda_sparse_matrix.cc b/internal/ceres/cuda_sparse_matrix.cc
index 7507b15..33685a4 100644
--- a/internal/ceres/cuda_sparse_matrix.cc
+++ b/internal/ceres/cuda_sparse_matrix.cc
@@ -185,7 +185,6 @@
 void CudaSparseMatrix::SpMv(cusparseOperation_t op,
                             const cusparseDnVecDescr_t& x,
                             const cusparseDnVecDescr_t& y) const {
-  size_t buffer_size = 0;
   const double alpha = 1.0;
   const double beta = 1.0;