Also removed ContentHandlerDefaultModelFor hook.
isTalkpageOccupied was mostly duplicating the existing functionality
already now that we're no longer using $wgFlowOccupy* globals but
$wgNamespaceContentModels. The additional check against redirects in there
didn't matter because redirects are wikitext content model anyway.
Meanwhile also removed msg flow-talk-taken-over, which is no longer
used (it was the content of the new revision before we started properly
doing it the content-model way)