statistics: fix conntrack regression caused by collectd 5.5.0

Collectd 5.5.0 introduced new data to conntrack plugin:
In addition to the number of tracked connections there is also
the static max conntrack value and the calculated use percentage.

Luci's conntrack plugin intrepretes "conntrack-max" as a new data instance
and includes it in the graph in addition to the real "conntrack" number.

Eliminate "max" from graph by specifying empty "" instance as data source.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>

applications/luci-app-statistics/luasrc/statistics/rrdtool/definitions/conntrack.lua

@@ -9,6 +9,10 @@ function rrdargs( graph, plugin, plugin_instance, dtype )
 		vlabel = "Count",
 		number_format = "%5.0lf",
 		data = {
+			-- collectd 5.5+: specify "" to exclude "max" instance
+			instances = {
+				conntrack = { "" }
+			},
 			sources = {
 				conntrack = { "value" }
 			},