deploy/cobbler: drop tcp_tw_recycle in sysctl.conf

deploy/cobbler: drop tcp_tw_recycle in sysctl.conf

Currently, tcp_tw_recycle is set explicitly in sysctl.conf when
installing OS by cobbler. This option is removed in kernel4.12,
current sysctl.conf breaks sysctl function and K8s deployment.

Actually, tcp_tw_recycle is considered broken and not recommended to
use. We may simply drop this option to be compatible with different
kernel versions.

References:
- https://vincent.bernat.im/en/blog/2014-tcp-time-wait-state-linux
- https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=4396e46187ca5070219b81773c4e65088dac50cc

Change-Id: I2cd89db3d423c811ba58eb5960f3cc718475be5f
Signed-off-by: Yibo Cai <yibo.cai@arm.com>