Remove an unneeded virtual method from EigenSparseCholesky.

Change-Id: I4f9d1730feb98b21db8dea9cc1357a365fb8fcb8

internal/ceres/eigensparse.h

diff --git a/internal/ceres/eigensparse.h b/internal/ceres/eigensparse.h
index 402bb5c..c73a811 100644
--- a/internal/ceres/eigensparse.h
+++ b/internal/ceres/eigensparse.h
@@ -54,8 +54,6 @@
   // SparseCholesky interface.
   virtual ~EigenSparseCholesky();
   virtual CompressedRowSparseMatrix::StorageType StorageType() const = 0;
-  virtual LinearSolverTerminationType Factorize(
-      const Eigen::SparseMatrix<double>& lhs, std::string* message) = 0;
   virtual LinearSolverTerminationType Solve(const double* rhs,
                                             double* solution,
                                             std::string* message) = 0;