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/control_loops/drivetrain/hybrid_ekf.h b/frc971/control_loops/drivetrain/hybrid_ekf.h
index 737c179..d3de332 100644
--- a/frc971/control_loops/drivetrain/hybrid_ekf.h
+++ b/frc971/control_loops/drivetrain/hybrid_ekf.h
@@ -741,7 +741,8 @@
     }
     if (obs->h == nullptr) {
       CHECK(obs->make_h != nullptr);
-      obs->h = CHECK_NOTNULL(obs->make_h->MakeExpectedObservations(*state, *P));
+      obs->h = obs->make_h->MakeExpectedObservations(*state, *P);
+      CHECK(obs->h != nullptr);
     }
     CorrectImpl(obs, state, P);
   }