Don't mark first feed read when deleting all items in search itemlist
Don't mark first feed read when deleting all items in search itemlist

`pos` is not configured for search itemlists so it has the default value
`0`. `v->get_ctrl()->mark_all_read(pos)` would mark all items in the
first feed read, even if the search results didn't include any of its
items.

newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,newsboat/newsboat,der-lyse/newsboat,der-lyse/newsboat,der-lyse/newsboat