Documentation/networking: delete orphaned 3c505.txt file.

In the commit 0e245dbaac9fa1c2fd0f4e2af7b9f6d874083a8b
("drivers/net: delete the 3Com 3c505/3c507 intel i825xx support")
we clobbered the 3c505 driver (over a year ago) along with other
abandoned ISA drivers.

However, this orphaned README file escaped detection at that
time, and has lived on until today. Get rid of it now.

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