sysctl/net.txt: delete reference to obsolete 2.4.x kernel

This random old kernel version has no bearing on the actual
file contents, and has not had for a long time.  Delete it.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>