device: fix memory leak
l3cd instances must be removed from the old l3cfg before calling
_cleanup_ip_pre(). Otherwise, _cleanup_ip_pre() unregisters them from
the device, and later _dev_l3_register_l3cds(self, l3cfg_old, FALSE, FALSE) does nothing because the device doesn't have any l3cd.
Previously the l3cds would linger in the l3cfg, keeping a reference to it and causing a memory leak; the leak was not detected by valgrind because the l3cfg was still referenced by the NMNetns.
Fixes: 58287cbc ('core: rework IP configuration in NetworkManager using layer 3 configuration') Fixes-test: @stable_mem_consumption2