Drop Platform::enabledOutputs()

At the moment, a platform should provide two output lists - one that lists all available outputs, and the other one that contains only enabled outputs. In general, this amounts to some boilerplate code and forces backends to be implemented in some certain way, which sometimes is inconvenient, e.g. if an output is disabled or enabled, it will be simpler if we only change Output::isEnabled(), otherwise we need to start accounting for corner cases such as the order in which Output::isEnabled() and Platform::enabledOutputs() are changed, etc.