rx: Remove duplicate out of order ACK check

rx: Remove duplicate out of order ACK check

Once we've moved the congestion window, there's no going back. So
any ACK packets that attempt to move the window backwards by including
a 'firstPacket' value earlier than the current window position must
be ignored.

However, we check (and ignore) these packets twice. Once in
rxi_ReceivePacket, which only checks in the client side case, and again
in rxi_ReceiveAckPacket, which has a more complete check that runs for
both client and server connections.

Remove the identical check from rxi_ReceivePacket in a continuing effort
to clean up this bit of code.

Change-Id: I090bc289848d0797860f46aec5877ad07fcc9b82
Reviewed-on: http://gerrit.openafs.org/8299
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>