drivers/net: delete intel 486 panther onboard ethernet support

This driver was specific to a "professional workstation" line
of products from around 1993 that used the i82596 ethernet chip
as an on-board ethernet solution.

With a 486 processor, and the premium top of the line model maxing
out at a clock speed of 50MHz, we can safely retire this support.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>