core: prevent multiple attempts to create default wired connection

Scenario:

  - have ethernet connection as unmanaged.
  - create (or have) a suitable profile for the connection.
  - make the device as managed. No default wired connection gets
    created.
  - delete the profile.

Note that NMManager does in manager_device_state_changed():

»···if (NM_IN_SET (new_state,
»···               NM_DEVICE_STATE_UNAVAILABLE,
»···               NM_DEVICE_STATE_DISCONNECTED))
»···»···nm_settings_device_added (priv->settings, device);

that means, when the device the next time goes through
UNAVAILABLE/DISCONNECTED states, we will suddenly create the
default "Wired connection 1" profile.

That doesn't seem right. When a device is suitable to have a
default-wired connection, we should only check once whether to
create it. We should not retry that later. The !no-auto-default
mechanism exists so we can start NetworkManager without a profile for
the device. It doesn't mean that we later one (after previously deciding
not to create a profile), we still create it.

https://bugzilla.redhat.com/show_bug.cgi?id=1687937

https://gitlab.freedesktop.org/NetworkManager/NetworkManager/-/merge_requests/450

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