hamachi: Delete TX checksumming code commented out since 1999

TX checksumming support has been ifdef commented out of this driver
for more than 10 years, and it makes references to aspects of the IPv4
stack from back then as well.

If someone has one of these rare cards and wants to properly resurrect
TX checksumming support, they can still get at this code in the
version control history.

Signed-off-by: David S. Miller <davem@davemloft.net>