Only call signal.Notify once during agent startup

Calling twice appears to have no adverse effects, however serves to
confuse as to what the semantics of such code may be! This seems like it
was probably introduced while resolving conflicts during the merge of
the fix for #2404.

command/agent/agent.go

@@ -385,7 +385,6 @@ func (c *cmd) run(args []string) int {
 
 	// wait for signal
 	signalCh := make(chan os.Signal, 10)
-	signal.Notify(signalCh, os.Interrupt, syscall.SIGTERM, syscall.SIGHUP)
 	signal.Notify(signalCh, os.Interrupt, syscall.SIGTERM, syscall.SIGHUP, syscall.SIGPIPE)
 
 	for {