Remove NETBSD_PF. NETBSD_PF is a flag that defines whether the pfil(9)
framework is available. pfil(9) has been in FreeBSD since FreeBSD 5
and according to svn log was first committed to HEAD in 2000, therefore
it is safe to say the check is no longer needed in FreeBSD.

pfil(9) first appeared in NetBSD 1.3 (hence the name NETBSD_PF).
Therefore it is safe to say that it is supported by every NetBSD system
today. The framework also exists in illumos.

As ipfilter code is shared and exchanged between FreeBSD and NetBSD, and
at some point in the future illumos too, and as all three platforms have
pfil(9), the redundant NETBSD_PF #defines and #ifdefs are removed.

MFC after: 1 week