Remove Platform::prepareShutdown() and Platform::areOutputsEnabled()
They are not needed anymore.
The main reason why Platform::prepareShutdown() was introduced is so that kwin doesn't try to access destroyed OutputDeviceInterface objects. However, !822 (merged) fixed that design issue, output objects can be used even after WaylandServer is torn down.
Platform::areOutputsEnabled() is unnecessary too. If an output is disabled, the compositor won't perform compositing on it. So, we don't gain anything from it.