Reduce number of dependencies drawn transitively by the xwiki-xml module

Right now xwiki-xml module has a Script service implementation which draws 4 more dependencies than needed for modules depending on xwiki-xml but not needing the script service implementation (the Rendering for example).

See http://tinycoke.com/_6rAfu3qTdTaeI/screen_shot_2011-02-28_at_10.25.09_am.png