ppp: remove more unnecessary kernel checks

ppp: remove more unnecessary kernel checks

The ppp package can support all features since Linux 4.7.0 kernel.
Therefore, most kernel version checks can pass unconditionally on
OpenWrt v18.06 and later version. This patch can reduce the size
of ppp package by approximately 2.5 KB.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>
Link: https://github.com/openwrt/openwrt/pull/16695
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
(cherry picked from commit 3dbe730080e70e96990a5e89cbe2035adcca8d4b)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>