wbb1_thread.polls is decremented twice when deleting posts

1. Create a new thread without a poll (polls = 0)
2. Create a reply with a poll (polls = 1)
3. Trash the reply (polls = 0 because of ThreadEditor::rebuildThreadData not counting trashed posts)
4. Delete the reply (polls = -1 because of PostAction::delete calling updateCounters for the poll count)

I suggest that the updateCounters call is replaced by a rebuildThreadData call, possibly the one a few lines above could just be moved down.