XWIKI-1454: Split XWiki's maven2 remote repository into 3: snapshots, releases and externals

* Added externals/ repository
* Removed definition of external repos as we don't need them anymore (now that we're asking users to define the repos in their settings.xml files)

git-svn-id: https://svn.xwiki.org/svnroot/xwiki/xwiki-platform/xwiki-tools/trunk@3811 f329d543-caf0-0310-9063-dda96c69346f