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/y2020/control_loops/drivetrain/localizer.cc b/y2020/control_loops/drivetrain/localizer.cc
index fabbb8e..0943b28 100644
--- a/y2020/control_loops/drivetrain/localizer.cc
+++ b/y2020/control_loops/drivetrain/localizer.cc
@@ -14,7 +14,8 @@
 // appropriate.
 Eigen::Matrix<float, 4, 4> FlatbufferToTransformationMatrix(
     const frc971::vision::sift::TransformationMatrix &flatbuffer) {
-  CHECK_EQ(16u, CHECK_NOTNULL(flatbuffer.data())->size());
+  CHECK(flatbuffer.data() != nullptr);
+  CHECK_EQ(16u, flatbuffer.data()->size());
   Eigen::Matrix<float, 4, 4> result;
   result.setIdentity();
   for (int row = 0; row < 4; ++row) {