net: Kill unuseful net/TUNABLE doc in kernel source

File net/TUNABLE has never be updated since git age.

For some tunable parameters which user can control with proc file-system,
They are all in ip-sysctl.txt doc.

For tunable parameters that only at compile time, no meaning to note them.

Signed-off-by: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>