nuke in_cksum_addword()

nuke in_cksum_addword()
don't we all love functions implemented in header files? was under #ifdef
_KERNEL at least.
incremental checksum updates don't really make sense any more, this is
incredibly hard to get right, and doesn't fit the way our kernel deals
with the checksums these days. consequently, nothing uses in_cksum_addword
any more.
was on tech for 3 months, tested by & discussed with many.