pf: Remove unused arguments from pf_send_tcp()

struct mbuf *replyto is not actually used (and only rarely provided).
The same applies to struct ifnet *ifp.

No functional change.

Reviewed by: mjg
MFC after: 1 week
Sponsored by: Modirum MDPay
Differential Revision: https://reviews.freebsd.org/D31136

(cherry picked from commit 295f2d939d960e2bdf5c1499da3eb41618be05e6)