mptcp: Remove unused mptcp_add_addr_ack from tcp_sock

The flag mptcp_add_addr_ack from struct tcp_sock was a remnant flag that is no
longer being used, so we removed it.

Signed-off-by: Cristina Ciocan <cristina.ciocan@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>