libnm/client: don't reset properties when interface goes away

In case the D-Bus interfaces start dropping off (typically all off them go
one by one when the object is being deleted), don't reset all the properties.

In particular, keep most properties around, only tear down "o" and "ao",
so that the object dependencies get torn down, but we still get enough
properties around to identify what the dead object was its heyday.

One example of where this is not good is when the device-removed signal
is emmitted, the device no longer has the ifname:

  $ nmcli monitor
  <quit NetworkManager>
  (null): device removed
  (null): device removed
58 jobs for lr/object-removal in 35 minutes and 12 seconds (queued for 3 seconds)