mptcp: remove data_ack and data_seq from tcp_out_options

We can access the source directly in mptcp_options_write, no need to
carry them through tcp_out_options.

Signed-off-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Christoph Paasch <christoph.paasch@uclouvain.be>