pf: remove redundant checks

pf_test() and pf_test6() drop IPv4-ICMP6 and IPv6-ICMP packets. Do
not do the same check in pf_test_rule() again.
ok henning

Reviewed by: zlei
Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, de5c2bfb74
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46591