Remove example 31 from the list of examples as it does not appear to display anything.
Remove example 31 from the list of examples as it does not appear to display anything.

svn path=/trunk/; revision=9339

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot