pf: rename pf_unlink_state() to pf_remove_state()

The OpenBSD rationale ('so the name does not collide with the statekey to inp
unlinking') doesn't apply to us, but it makes future imports easier if we follow
this rename.

Obtained from: OpenBSD, bluhm <bluhm@openbsd.org>, acb93c8a64
Sponsored by: Rubicon Communications, LLC ("Netgate")