bench_spawn.py: remove unused import

--HG--
extra : transplant_source : %A0%DB%8C%FB%C0%F5%2C%7D%18%9D%1D%DB%BB2%7D%0B%D6%1B%13%DE

greentest/bench_spawn.py

@@ -3,7 +3,6 @@
 import sys
 import os
 import random
-import traceback
 from time import time