Don't clean the LESS cache on restarts

Right now the filesystem cache for LESS is purged at each restart (see http://extensions.xwiki.org/xwiki/bin/view/Extension/LESS+Module#HPerformances).

I think we shouldn't clean it by default and instead whenever there's a change in the cache format, mention in the release notes that the cache should be deleted (same as we do for SOLR).

This would win us a lot of perf when restarting XWiki.