security/expiretable: Fix build on 13+

get_states() is the reason it stopped building on FreeBSD 13 and newer.
Retire this function since it's not used anywhere and get the build
fixed.

PR: 253547
Reported by: mike@sentex.net
Reviewed by: kp
Approved by: maintainer timeout (2 weeks)
Obtained from: pfSense
MFH: 2021Q2
Sponsored by: Rubicon Communications, LLC ("Netgate")