pf: Remove unused variable rt_listid from struct pf_krule

Reviewed by: donner
MFC after: 4 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D29639

(cherry picked from commit 4967f672ef3095300fe74a9d1ae873d0897cc0a5)