Remove check_running method of exec backend.

`check_running` of `Exec` backend checks whether the service is running or not in the following way:

1. Check whether the service is running or not (e.g. `service status servicename` in ubuntu)
2. Check whether a process of the service is running or not (e.g. `ps aux | grep servicename`)

I removed Exec#check_running because this behaviour is different from the one of `Ssh` backend and it's confusing.