Cleanup setup-ip script a bit

There is no longer a case where $INTERFACE is not set.

network/setup-ip

@@ -181,7 +181,6 @@ if [ -z "$INTERFACE" ]; then
     exit 1
 fi
 
-if [ -n "$INTERFACE" ]; then
 if [ "$ACTION" == "add" ]; then
     MAC="$(get_mac_from_iface "$INTERFACE")"
     if [ -n "$MAC" ]; then
@@ -251,5 +250,3 @@ if [ -n "$INTERFACE" ]; then
     echo "Invalid action '$ACTION'" >&2
     exit 1
 fi
-
-fi