libnm: drop unused property setter NM_DEVICE_MANAGED

NM_DEVICE_MANAGED was intended to work like NM_DEVICE_AUTOCONNECT:
namely it would call the D-Bus property setter synchronously.

But such behavior is horrendous, we certainly don't want blocking calls
during a property getter.

Luckily this one instance was unused and never worked as the property
was marked as G_PARAM_READABLE. Just drop the setter.