dynamicplaylist: Ensure subclasses handle virtual calls in dtor.

LGTM points out that DynamicPlaylist's destructor calls a function which
indirectly leads to a virtual call (updateItems) in a potential subclass
(SearchPlaylist). By C++ rules, SearchPlaylist has ceased to be a
SearchPlaylist by the time DynamicPlaylist::~DynamicPlaylist() begins,
so the virtual call would never actually be dispatched to
SearchPlaylist.

Fix by having SearchPlaylist do its own updating on destruction.