network.subr: unobsolete gif_interfaces

There are cases when gif_interfaces cannot be replaced
with cloned_interfaces, such as tunnels with external IPv6 addresses
and internal IPv4 or vice versa. Such configuration requires
extra invocation of ifconfig(8) and supported with gif_interfaces only.

MFC after:	1 week

libexec/rc/network.subr

@@ -1369,9 +1369,6 @@ clone_up()
 			fi
 		esac
 	done
-	if [ -n "$gif_interfaces" ]; then
-		warn "\$gif_interfaces is obsolete.  Use \$cloned_interfaces instead."
-	fi
 	for ifn in ${gif_interfaces}; do
 		# Parse ifn:ifopt.
 		OIFS=$IFS; IFS=:; set -- $ifn; ifn=$1; ifopt=$2; IFS=$OIFS