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/wpilib/talonfx.cc b/frc971/wpilib/talonfx.cc
index 152339e..4338cfd 100644
--- a/frc971/wpilib/talonfx.cc
+++ b/frc971/wpilib/talonfx.cc
@@ -21,7 +21,7 @@
   // device temp is not timesynced so don't add it to the list of signals
   device_temp_.SetUpdateFrequency(kCANUpdateFreqHz);
 
-  CHECK_NOTNULL(signals);
+  CHECK(signals != nullptr);
 
   supply_voltage_.SetUpdateFrequency(kCANUpdateFreqHz);
   signals->push_back(&supply_voltage_);