Delete obsolete HTML documentation files from TracOnlineDocumentation

[jjr8]

The build system automatically generates HTML documentation files for new modules, classes, methods, etc. and stores them in the PythonPackage?/dist/TracOnlineDocumentation tree. But it does not automatically remove HTML files when a module, class or whatever is removed from the source code. For example, the ArcGISRaster class used to have a method called CopyRasters, but it was renamed to CopyList. The TracOnlineDocumentation tree still has HTML files for the CopyRasters method. These files, and all others that refer to obsolete things, should be periodically scrubbed. Ideally the build script would automatically delete them, but this might require a call to a subversion API.