pf: remove pd_refs from pfsync

It only served to complicate cleanup, and added no value.

While here drop packets in pfsync_defer_tmo() if we don't have a syncif,
rather than just leaving them on the queue.

Reviewed by: markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D39248

(cherry picked from commit 27b23cdec9d1255e014ea0decac42a5be8d00735)