The function is unused. It would require redesign to work with
future changes, and since it's unused, just drop it.

The long reasoning is:

    Currently, a refresh-all is tied to an NMPObjectType. However, with
    NMPObjectRoutingRule (for policy-routing-rules) that will no longer
    be the case.

    That is because NMPObjectRoutingRule will be one object type for
    AF_INET and AF_INET6. Contrary to IPv4 addresses and routes, where
    there are two sets of NMPObject types.

    The reason is, that it's preferable to treat IPv4 and IPv6 objects
    similarly, that is: as the same type with an address family property.

    That also follows netlink, which uses RTM_GET* messages for both
    address families, and the address family is expressed inside the
    message.

    But then an API like nm_platform_refresh_all() makes little sense,
    it would require at least an addr_family argument. But since the
    API is unused, just drop it.