network: drop invalid assertion

The link may not have corresponding .network file.
Note that in that case, link_ipv4ll_enabled() and link_dhcp4_enabled()
returns false. So, it is safe to drop the assertion.

Fixes #12422.

src/network/networkd-link.c

@@ -1854,7 +1854,6 @@ static int link_acquire_ipv4_conf(Link *link) {
         int r;
 
         assert(link);
-        assert(link->network);
         assert(link->manager);
         assert(link->manager->event);