libnm: don't check if the agent is still registered when unregistering

libnm: don't check if the agent is still registered when unregistering

The name_owner_chagned() unregisters the agent if NetworkManager goes away and
nmc_cleanup() also tries to unregister an agent, resulting in an assertion
failure:

  # nmcli c up conn666
  <daemon terminates>
  Error: Connection activation failed: Message recipient disconnected from message bus without replying
  (process:8746): libnm-CRITICAL **: nm_secret_agent_old_unregister: assertion 'priv->registered == TRUE' failed

_internal_unregister() already contains a priv->registered check and raising an
error on duplicate unregister attempt from a daemon after a restart is not a
problem either, since nmc_cleanup() doesn't care about the error returned
on teardown anyway.