The following procedure leaves an NMActiveConnection around for a deactivated
device, which causes errors in libnm-glib clients when they cannot create the
GObject for the non-existent device of the AC.

1) allow a device which can assume connections to be activated
2) stop NM, which should leave the device's IP configuration up
3) start NM and allow it to assume the device's existing connection
4) remove the device, either by unplugging it or 'rmmod'

The device is removed by nm-manager.c::remove_device(), but the device object
is not moved to UNMANAGED state, leaving the NMActiveConnection completely
unaware the device has gone away.

The nm-manager.c::remove_device() code did not correctly handle moving a
forcibly removed (eg, by unplugging or 'ip link del' or 'rmmod') device to
the UNMANAGED state when the device was active with an assumed connection.
To fix this, make the conditions when the device should be deactivated
on removal much more explicit.

A device should be deactivated on removal if:

1) it is forcibly removed, eg by the kernel network interface being
removed due to 'ip link del' or hotplugging, or internally by NM due
to a parent WWAN interface taking priority over a WWAN ethernet interface

2) if the device cannot assume connections, in which case NetworkManager
must have activated the device and since we cannot assume the connection
on restart, we should deactivate it

3) if the device is not activated, to ensure that its IPv6 parameters
and other things get reset to the pre-NetworkManager values

https://bugzilla.gnome.org/show_bug.cgi?id=729833