Remove unused fields in siftr_stats. Thus, update the man page as well.

Summary: Remove unused fields in siftr_stats. Thus, update the man page as well.

Test Plan: Tested in Emulab testbed.

Reviewers: rscheff, tuexen
Approved by: rscheff, tuexen
Subscribers: imp, melifaro, glebius
Differential Revision: https://reviews.freebsd.org/D39776