Do not print notifications for new threads

This information is hardly going to be useful to a user of Delve and if
the user really needs information on new threads they can simply using
the provided commands to list and switch between threads.

proctl/proctl_darwin.go

@@ -111,7 +111,6 @@ func (dbp *DebuggedProcess) addThread(port int, attach bool) (*ThreadContext, er
 	if thread, ok := dbp.Threads[port]; ok {
 		return thread, nil
 	}
-	fmt.Println("new thread spawned", port)
 	thread := &ThreadContext{
 		Id:      port,
 		Process: dbp,

proctl/proctl_linux.go

@@ -65,7 +65,6 @@ func (dbp *DebuggedProcess) addThread(tid int, attach bool) (*ThreadContext, err
 	if thread, ok := dbp.Threads[tid]; ok {
 		return thread, nil
 	}
-	fmt.Println("new thread spawned", tid)
 
 	if attach {
 		err := sys.PtraceAttach(tid)