Remove TalkpageManager::isTalkpageOccupied


Remove TalkpageManager::isTalkpageOccupied

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)

Bug: T105574
Change-Id: I344769c1fe7616218a518ded02e8aa9537a0ebce