Remove the redundant ipfilter IPv6 rc rules load.

As of ipfilter 5.1.2 the IPv4 and IPv6 rules tables have been merged.
The ipf(8) -6 option has been a NOP since then. Currently the additional
ipf -6 load statement in rc.d/ipfilter simply added the second ipfilter
rules file to the table already populated by the previous ipf command.
Plenty of time has passed since ipfilter 5.1.2 was imported. It is time to
remove the option from rc.conf and the rc script.

Differential Revision:	https://reviews.freebsd.org/D28615

libexec/rc/rc.conf

@@ -546,9 +546,6 @@ stf_interface_ipv6_ifid="0:0:0:1"	# IPv6 interface id for stf0.
 stf_interface_ipv6_slaid="0000"	# IPv6 Site Level Aggregator for stf0
 ipv6_ipv4mapping="NO"		# Set to "YES" to enable IPv4 mapped IPv6 addr
 				# communication. (like ::ffff:a.b.c.d)
-ipv6_ipfilter_rules="/etc/ipf6.rules"	# rules definition file for ipfilter,
-					# see /usr/src/contrib/ipfilter/rules
-					# for examples
 ip6addrctl_enable="YES"	# Set to YES to enable default address selection
 ip6addrctl_verbose="NO"	# Set to YES to enable verbose configuration messages
 ip6addrctl_policy="AUTO"	# A pre-defined address selection policy

libexec/rc/rc.d/ipfilter

@@ -14,7 +14,7 @@ name="ipfilter"
 desc="IP packet filter"
 rcvar="ipfilter_enable"
 load_rc_config $name
-stop_precmd="test -f ${ipfilter_rules} -o -f ${ipv6_ipfilter_rules}"
+stop_precmd="test -f ${ipfilter_rules}"
 
 start_precmd="$stop_precmd"
 start_cmd="ipfilter_start"
@@ -39,10 +39,6 @@ ipfilter_start()
 		${ipfilter_program:-/sbin/ipf} \
 		    -f "${ipfilter_rules}" ${ipfilter_flags}
 	fi
-	if [ -r "${ipv6_ipfilter_rules}" ]; then
-		${ipfilter_program:-/sbin/ipf} -6 \
-		    -f "${ipv6_ipfilter_rules}" ${ipfilter_flags}
-	fi
 }
 
 ipfilter_stop()
@@ -67,13 +63,6 @@ ipfilter_reload()
 			err 1 'Load of rules into alternate set failed; aborting reload'
 		fi
 	fi
-	if [ -r "${ipv6_ipfilter_rules}" ]; then
-		${ipfilter_program:-/sbin/ipf} -I -6 \
-		    -f "${ipv6_ipfilter_rules}" ${ipfilter_flags}
-		if [ $? -ne 0 ]; then
-			err 1 'Load of IPv6 rules into alternate set failed; aborting reload'
-		fi
-	fi
 	${ipfilter_program:-/sbin/ipf} -s
 
 }