DropdownInputWidget: Fix mixing in TitledElement twice
DropdownInputWidget: Fix mixing in TitledElement twice

TitledElement is already mixed in by way of the parent class InputWidget.
The title is set on the invisible <select> element. Instead of mixing
it in again, just override where the title goes.

Follow-up to 87f1dcfa283efe8c43cbf040de136e63e58ce819.

Change-Id: I422e925f55051861a9cc6648563c2425fbd130ea

wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui,wikimedia/oojs-ui