drivers/net: remove all references to obsolete Ethernet-HOWTO

This howto made sense in the 1990s when users had to manually configure
ISA cards with jumpers or vendor utilities, but with the implementation
of PCI it became increasingly less and less relevant, to the point where
it has been well over a decade since I last updated it.  And there is
no value in anyone else taking over updating it either.

However the references to it continue to spread as boiler plate text
from one Kconfig file into the next.  We are not doing end users any
favours by pointing them at this old document, so lets kill it with
fire, once and for all, to hopefully stop any further spread.

No code is changed in this commit, just Kconfig help text.

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