include/linux/i82593.h: Remove unused header

The header file include/linux/i82593.h does not seem to be used
anywhere. It was orphaned by 8a594170

 "drivers/net: delete intel
i825xx based znet notebook driver". Remove it.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>