Remove redundant declaration for tcp_dooptions, similar to r302576

netinet/tcp_var.h already defines this function

Differential Revision: https://reviews.freebsd.org/D7189
MFC after: 1 week
PR: 209920
Reported by: Mark Millard <markmi@dsl-only.net>
Reviewed by: np
Tested with: clang 3.8.0, gcc 4.2.1, gcc 5.3.0
Sponsored by: EMC / Isilon Storage Division