Remove "from __future__ import print"

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

Fixes #652

examples/simpleperf.py

@@ -9,7 +9,6 @@ iperf will hang indefinitely if the TCP handshake fails
 to complete.
 """
 
-from __future__ import print_function
 
 from mininet.topo import Topo
 from mininet.net import Mininet