pf: Remove redundant kif != NULL checks

pf_kkif_free() already checks for NULL, so we don't have to check before
we call it.

Reviewed by: melifaro@
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29195

(cherry picked from commit 913e7dc3e0eb7df78ec0e7ecc7dd160a316a3ac6)