Remove unnecessary #include

Change-Id: I310b71694ce775c5f79c15aebeba8fd3bf603c06
diff --git a/y2017/constants.cc b/y2017/constants.cc
index 1a2924d..c11dbcc 100644
--- a/y2017/constants.cc
+++ b/y2017/constants.cc
@@ -13,7 +13,6 @@
 #include "aos/logging/logging.h"
 #include "aos/mutex/mutex.h"
 #include "aos/network/team_number.h"
-#include "aos/once.h"
 #include "absl/base/call_once.h"
 
 #include "y2017/control_loops/drivetrain/drivetrain_dog_motor_plant.h"