udhcpc: remove workaround for bugs in Win98 dhcp server ("MSFT 98" vendor string)

udhcpc: remove workaround for bugs in Win98 dhcp server ("MSFT 98" vendor string)

Stats for last three commits:

function                                             old     new   delta
udhcpc_main                                         2635    2646     +11
udhcp_recv_raw_packet                                425     414     -11
udhcp_recv_kernel_packet                             210     134     -76
packed_usage                                       28940   28857     -83
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 1/3 up/down: 11/-170)          Total: -159 bytes
   text    data     bss     dec     hex filename
 879524     493    7584  887601   d8b31 busybox_old
 879340     493    7584  887417   d8a79 busybox_unstripped

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