Remove commented debug statement that used old API.

Author: Carreau

jupyter_client/channels.py

@@ -142,7 +142,6 @@ def run(self):
                 continue
 
             since_last_heartbeat = 0.0
-            # io.rprint('Ping from HB channel') # dbg
             # no need to catch EFSM here, because the previous event was
             # either a recv or connect, which cannot be followed by EFSM
             self.socket.send(b'ping')