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

60 jobs for bg/mem in 45 minutes and 9 seconds (queued for 4 seconds)