Remove usage of CHECK_NOTNULL

We want to switch to absl logging instead of glog.  gtest and ceres are
going there, and we already have absl as a dependency.  ABSL doesn't
have CHECK_NOTNULL, and we can move things over in an easier to review
fashion.

Change-Id: Ifd9a11ec34a2357cec43f88dba015db9c28ed2cf
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/vision/target_mapper.cc b/frc971/vision/target_mapper.cc
index 57ab6e4..f245f37 100644
--- a/frc971/vision/target_mapper.cc
+++ b/frc971/vision/target_mapper.cc
@@ -467,7 +467,7 @@
 
 // Taken from ceres/examples/slam/pose_graph_3d/pose_graph_3d.cc
 bool TargetMapper::SolveOptimizationProblem(ceres::Problem *problem) {
-  CHECK_NOTNULL(problem);
+  CHECK(problem != nullptr);
 
   ceres::Solver::Options options;
   options.max_num_iterations = FLAGS_max_num_iterations;