Remove "from __future__ import print"

This was well-intentioned, but it causes more trouble
than it's worth.

Fixes #652

bin/mn

@@ -11,7 +11,6 @@ Example to pull custom params (topo, switch, etc.) from a file:
   sudo mn --custom ~/mininet/custom/custom_example.py
 """
 
-from __future__ import print_function
 from optparse import OptionParser
 import os
 import sys