pf: remove DIOCGETSTATESNV

While nvlists are very useful in maximising flexibility for future
extensions their performance is simply unacceptably bad for the
getstates feature, where we can easily want to export a million states
or more.

The DIOCGETSTATESNV call has been MFCd, but has not hit a release on any
branch, so we can still remove it everywhere.

Reviewed by: mjg
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D31099

(cherry picked from commit b69019c14cd89a5e699bc6e69941aec5b1041b1e)