Don't bother checking for negative delay/jitter

It's kind of pointless. However technically the argument should
be a number and not a string - it's always in ms.

mininet/link.py

@@ -285,11 +285,7 @@ class TCIntf( Intf ):
                    loss=None, max_queue_size=None ):
         "Internal method: return tc commands for delay and loss"
         cmds = []
-        if delay and delay < 0:
-            error( 'Negative delay', delay, '\n' )
-        elif jitter and jitter < 0:
-            error( 'Negative jitter', jitter, '\n' )
-        elif loss and ( loss < 0 or loss > 100 ):
+        if loss and ( loss < 0 or loss > 100 ):
             error( 'Bad loss percentage', loss, '%%\n' )
         else:
             # Delay/jitter/loss/max queue size