net: Remove net_tcp_prepare_data_segment

This is dead code in the tree as it stands, and the upcoming rework is
so changed as to make trying to evolve it in place impossible.  Also
removes some temporary tracking fields in the net_tcp and tcp_segment
structs which had been intended for this implementation.

Change-Id: I59fe00cd2d5c3e48bc5d38d3d096d9ff72fcdccf
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>