Remove useless overrides of QAbstractFileEngine methods from the derived
classes. Also remove "This virtual function must be reimplemented by
all subclasses" passages from the QAbstractFileEngine's documentation.
There are pure virtual methods for such use cases. QAbstractFileEngine
already contains useful defaults for classes not supporting all the
functionality.
Change-Id: Ia25965854f3809b15d7502da3749cc2f3414bbc3
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>