cleanup duplicate function

selfdrive/controls/lib/radar_helpers.py

@@ -1,3 +1,4 @@
+from common.numpy_fast import mean
 from common.kalman.simple_kalman import KF1D
 from selfdrive.config import RADAR_TO_CAMERA
 
@@ -54,9 +55,6 @@ class Track():
     self.aLeadK = aLeadK
     self.aLeadTau = aLeadTau
 
-def mean(l):
-  return sum(l) / len(l)
-
 
 class Cluster():
   def __init__(self):