luci-app-statistics: Remove usually empty std.dev. graph from ping page

Remove the usually empty standard deviation graph from the ping page.

The graph is empty for most users as collectd measures standard deviation
of individual pings inside the general interval of statistics collection.
Default setting for both ping interval and general collection interval
is 30s in Luci statistics, meaning just 1 ping per interval, which
leads to empty graph.

(To provide relveant data, the ping interval should be 1/4-1/5 of the
general collection interval. Even then the graph does not look very
informative due to different scaling than the latency graph.)

Note that this commit does not change collectd itself, which continues
to collect and calculate also the std.dev. data, which can be fetched
with 'rrdtool' if needed.

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