Documentation/sysctl/net.txt: fix (attribute removal).

This patch removes mentioning the sysfsf net_device weight attribute
(class/net/<device>/weight)
in Documentation/sysctl/net.txt, since the net sysfs weight attribute
was removed by the following patch:

[NET]: Make NAPI polling independent of struct net_device objects
 bea3348e

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>