pf: quiet -Wredundant-decls for pf_get_ruleset_number

In e86bddea9fe62d5093a1942cf21950b3c5ca62e5 sys/netpfil/pf/pf.h grew a
declaration of pf_get_ruleset_number. Now delete the old declaration
from sys/net/pfvar.h.

Reviewed by: kp
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D28081