udhcp: do not setlinebuf(stdout), we don't print to stdout anymore

udhcp: do not setlinebuf(stdout), we don't print to stdout anymore

Since bb_info_msg() was eliminated type of buffering on stdout is not important

function                                             old     new   delta
udhcpd_main                                         1463    1451     -12
udhcpc_main                                         2735    2723     -12
setlinebuf                                            19       -     -19
------------------------------------------------------------------------------
(add/remove: 0/2 grow/shrink: 0/2 up/down: 0/-43)             Total: -43 bytes

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