Cleanup: Remove unused variables from tcp_skb_cb

Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>
Fix: Fully fallback to infinite, if first data is without DSS

We have to do seamless bidirectional fallback, when the first data is without
DSS.
Bidirectional fallback has been introduced in c00e8536517 (Final fix to
infinite mapping)

Reported-by: Krishna Khanal <erkrishna.khanal@gmail.com>
Tested-by: Krishna Khanal <erkrishna.khanal@gmail.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>