Remove "xwiki.action.import.xar.usefilter" configuration property

This config property was introduced when the new export using filter stream was still new. That was in XWiki 6.2 and it's now been stable for a long time. See https://forum.xwiki.org/t/deprecate-legacify-old-xar-import-classes/11300 for more context.

It can have some security implications and thus the idea is to remove it from all supported branches.