Werk #399: check_ftp: cleanup configuration via WATO, remove Hostname field

The check_ftp active check does not longer need nor allow to specify a host name. The host name is always present: the host the check is mapped onto. Also the specification of an alternative host name would not be consistent with any of the other networking checks.

If you need an FTP check on a specific host then please add that host to the monitoring. Exceptions can alsway be done via custom_checks.

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