Performance test graph generation now refreshes the html directory [reviewed by Sung]

Alterred the genGraphs script so that it removes the stale html directory and
files before creating the new graph files.  In verbose mode, it will tell you
the specific directories it is deleting and creating.



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@22158 3a8e244f-b0f2-452b-bcba-4c88e055c3ca