Minor clean up in the htdocs directory.

Remove the `CAPA_Graphics` link.  I missed this when everything else
CAPA related was removed before.  (grep doesn't show it!)

Also remove the `crossdomain.xml` file.  I don't think this is not used
with the current approaches for serving webwork2, and if it is, it is
most likely a security vulnerability.

Author: drgrice1