ppp: remove discovery phase timeout hack patch

In the original code, the entire time delay of the discovery phase
is only 5+5x2+5x2x2 = 35s. Increasing timeout may be necessary if
discovery phase fails on first attempt. There is a chance to fix
the "Timeout waiting for PADO packets" issue by removing this patch.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: openwrt#16605
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>