luci-statistics: fix uhttpd log spam from extra png close

Fix log spam that uhttpd logging change has brought to surface.

Sun Mar  1 11:10:42 2015 daemon.err uhttpd[2293]: /usr/lib/lua/luci/dispatcher.lua:433: Failed to execute call dispatcher target for entry '/admin/statistics/graph/conntrack'.
Sun Mar  1 11:10:42 2015 daemon.err uhttpd[2293]: The called action terminated with an exception:
Sun Mar  1 11:10:42 2015 daemon.err uhttpd[2293]: .../luci/controller/luci_statistics/luci_statistics.lua:153: attempt to use a closed file

Closing the png file explicitly on line 153 is unnecessary, as
ltn12's source.file method already closes the file after use.
https://github.com/openwrt/luci/blob/master/modules/luci-base/luasrc/ltn12.lua#L119

More info at https://dev.openwrt.org/ticket/19083#comment:2

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