BaseKey: remove enabled property to avoid shadowing Item's

While perhaps not that important for BaseKey, for which child items
might not make a whole lot of sense, it shouldn't be necessary to
shadow Item's enabled property.

Task-number: QTBUG-55754
Change-Id: I6746b55cdda1947006f1d052b613a7e38f01fac2
Reviewed-by: Jarkko Koivikko <jarkko.koivikko@code-q.fi>
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>