pf.4/pf.conf.5: Remove unused sysctl

"We never actually added the sysctl. The limit was implemented, and the
sysctl to tune it was documented but never added (so the limit is there,
but is always 16 and cannot be changed)." ~kp

MFC after: 3 days
Fixes: 339a1977c324 (pf: Add a systl to limit work)
Reported by: kp
Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D50859