Remove size assert in QMultiHash::take()

It doesn't exist in every instantiation of the template; this becomes
evident when attempting to build QQuickListView:

error: 'using Chain = struct QHashPrivate::MultiNodeChain<FxViewItem*>'
  {aka 'struct QHashPrivate::MultiNodeChain<FxViewItem*>'} has no member named 'size'

Change-Id: I24748c15f3995083eec06d86a3c0dced2209c303
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qhash.h

@@ -1318,7 +1318,6 @@ public:
         auto it = d->find(key);
         if (it.isUnused())
             return T();
-        Q_ASSERT(it.node()->value.size());
         Chain *e = it.node()->value;
         Q_ASSERT(e);
         if (!e->next)