Remove redundant declaration for tcp_dooptions, similar to r302576
netinet/tcp_var.h already defines this function
MFC after: 1 week
PR: 209920
Reported by: Mark Millard <markmi@dsl-only.net>
Tested with: clang 3.8.0, gcc 4.2.1, gcc 5.3.0
Sponsored by: EMC / Isilon Storage Division