tap_int.h: remove repeating NETWORK_SCRIPT defines

DEFAULT_NETWORK_SCRIPT and DEFAULT_NETWORK_DOWN_SCRIPT
have been defined in net/net.h included in
tap.c, which is the only C file that using those two macro.
Let's remove the repeating macroinstruction.

Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>