system/device_table.txt: drop unused ifupdown post-up.d / pre-down.d directories

system/device_table.txt: drop unused ifupdown post-up.d / pre-down.d directories

These are not used by the busybox or Debian ifupdown implementation. Instead
if-up.d is run after the per-interface up tasks (E.G. post-up) and
if-down.d is run before the per-interface down tasks (E.G. pre-down).

So drop them as they only cause confusion.

grep execute_all output/build/busybox-1.23.2/networking/ifupdown.c
static int execute_all(struct interface_defn_t *ifd, const char *opt)
        if (!execute_all(iface, "pre-up")) return 0;
        if (!execute_all(iface, "up")) return 0;
        if (!execute_all(iface, "down")) return 0;
        if (!execute_all(iface, "post-down")) return 0;

grep execute_script output/build/ifupdown-0.7.49/execute.c
int execute_scripts(interface_defn * ifd, execfn * exec, char *opt)
    if (!execute_scripts(iface, doit, "pre-up"))
    if (!execute_scripts(iface, doit, "up"))
    if (!execute_scripts(iface, doit, "down"))
    if (!execute_scripts(iface, doit, "post-down"))

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>