lxc: remove legacy cgroups from common.conf

I am unaware of any kernel currently provided that retains cgroup v1 support.
This patch removes these lines in /usr/share/lxc/config/common.conf to allow
for error free usage.

Providing common.conf as-is will result in failure to start. One solution is to
comment out the legacy lines.[1] This requires users to either provide a custom
version of this file on their builds or to manually edit it with each update.

Since many do not build their own, the first option is not available to them.
Manually editing the file with each update will cause a failure to start
containers set to auto-start upon rebooting into the update.

1. https://forum.openwrt.org/t/openwrt-arm64-quick-lxc-howto-guide-lms-in-debian-system-in-lxc-container/99835



Signed-off-by: John Audia <graysky@archlinux.us>