remove the use of DCHECK_NOTNULL from numeric_diff.h

Change-Id: I7fc258812b1fe4aa727eaeb9b9b834d04a631abd

include/ceres/internal/numeric_diff.h

diff --git a/include/ceres/internal/numeric_diff.h b/include/ceres/internal/numeric_diff.h
index 09c94b0..8851bc7 100644
--- a/include/ceres/internal/numeric_diff.h
+++ b/include/ceres/internal/numeric_diff.h
@@ -72,7 +72,7 @@
     using Eigen::RowMajor;
     using Eigen::ColMajor;
 
-    DCHECK_NOTNULL(jacobian);
+    DCHECK(jacobian);
 
     const int num_residuals_internal =
         (kNumResiduals != ceres::DYNAMIC ? kNumResiduals : num_residuals);