[Server] suppress warning when hatohol terminates in foreground mode

Because following error displayed when hatohol server terminates in foreground mode:
```log
[ERR] <main.cc:108> Failed to remove pid file: /tmp/hatohol.pid
```

In foreground mode, hatohol server doesn't create pidfile.
I've selected following strategy suggested by kz0817:

* Call pidFilePath.clear() when the case that doesn't call daemonize().