networkd: do not touch link_messages when expiring routes

networkd: do not touch link_messages when expiring routes

link_messages is used during link configuration to advance the link
state machine through SETTING_ADDRESSES -> SETTING_ROUTES -> CONFIGURED.

If a route expires in the middle of this, it is possible for
link_messages to hit zero inside route_expire_callback, rather than in
route_handler or address_handler where it would trigger the next step in
configuration. Should this happen, the link will not complete
configuration, and it may not have its static routes configured.

Since route_expire_callback does not need to do anything once the
expired route has been removed from the kernel, it is safe to simply not
account for the netlink request.