pf: Simplify cleanup

We can now counter_u64_free(NULL), so remove the checks.

MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29190

(cherry picked from commit 28dc2c954f5096ae594ed5cd7a83d66ce4bf1ded)