remove Documentation/networking/net-modules.txt

According to git, the only one who touched this file during the last
5 years was me when removing drivers...

modinfo offers less ancient information.

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Paul Gortmaker <paul.gortmaker@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>