networking/cs89x0.txt: delete stale information about hand patching

Output of a git grep happened to make me look into this file, and
I found instructions about how to hand patch (without using patch)
the driver into the kernel tree.

Since the driver has been a part of the mainline kernel for years,
we can dump this whole section.  Fortunately it doesn't even cause
a renumbering of the sections to do so.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>