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/aos/util/config_validator_lib.cc b/aos/util/config_validator_lib.cc
index eb5205a..4347017 100644
--- a/aos/util/config_validator_lib.cc
+++ b/aos/util/config_validator_lib.cc
@@ -177,8 +177,10 @@
               }
               // TODO(james): This will be overly noisy, as it ends up
               // CHECK-failing.
-              required_timestamp_channels.insert(CHECK_NOTNULL(
-                  timestamp_finder.ForChannel(channel, connection)));
+              const Channel *found_channel =
+                  timestamp_finder.ForChannel(channel, connection);
+              CHECK(found_channel != nullptr);
+              required_timestamp_channels.insert(found_channel);
               break;
           }
         }