delete: do not exit if keytab cannot be read
Reading the keytab is not required when deleting a host object in AD. It is only needed in the case where the host was added with a manual set NetBIOS name (--computer-name option) which does not match the short hostname and no computer name was given at the delete-computer command line.
Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1840752