pfctl: pfik_ifp is always NULL

The pfik_ifp field is not provided by the kernel, it is always NULL. Do not
check for it. This caused us to not clear the skip flag on interfaces, leading
to unexpected behaviour when a 'set skip' was removed.

PR: 280834
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46311