udhcpc: remove code which requires server ID to be on local network

udhcpc: remove code which requires server ID to be on local network

This reverts "udhcpc: paranoia when using kernel UDP mode
for sending renew: server ID may be bogus".

Users complain that they do have servers behind routers
(with DHCP relays).

function                                             old     new   delta
send_packet                                          168     166      -2
bcast_or_ucast                                        25      23      -2
udhcp_send_kernel_packet                             301     295      -6
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-10)             Total: -10 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>