rx: Remove unused ACKHACK macro

rx: Remove unused ACKHACK macro

rx_globals.h had a macro called ACKHACK which at one point looks like
it was used to determine whether to send an ACK or not. We now have a
more sophisticate mechanism for determining this, and the old macro
doesn't seem to have been used in a long time. So, remove it to avoid
confusion.

Change-Id: I9b17cdca69c903c7a952489791088f2899ac7e60
Reviewed-on: http://gerrit.openafs.org/7000
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@secure-endpoints.com>