manager: don't allow removing a device without traversing proper states

There was a special case for ensuring that the device's configuration doesn't disappear when some factory is too late at recognizing the device is just a component of another one.

It was always a bad idea. If the device already had an active connection (such as for a generated default wired connection), it would remain around, with a dangling reference to the device.

This effectively reverts commit 5ad69cb2 ('core: remove child devices without deconfiguring them (bgo #738479)'). It's okay to do so, because we now wouldn't deconfigure the device upon its removal anyway.