network: remove stale function

network: remove stale function

networkMigrateStateFiles was added nearly 5 years ago when the network
state directory was moved from /var/lib/libvirt to /var/run/libvirt
just prior to libvirt-1.2.4). It was only required to maintain proper
state information for networks that were active during an upgrade that
didn't involve rebooting the host. At this point the likelyhood of
anyone upgrading their libvirt from pre-1.2.4 directly to 5.0.0 or
later *without rebooting the host* is probably so close to 0 that no
properly informed bookie would take *any* odds on it happening, so it
seems appropriate to remove this pointless code.

Signed-off-by: Laine Stump <laine@laine.org>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>