Drop QFile::fileEngine

Drop QFile::fileEngine

The function was already marked as internal.

By moving access to the underlying file engine to private API this
supports the effort to completely drop file engines from public API. In
the future, the goal is to completely drop the file-engine abstraction
as it exists today.

Change-Id: I332fa56e70c87e83c1e08bb9f75e04df7c93fec7
Reviewed-by: Jonas Gastal <jgastal@profusion.mobi>
Reviewed-by: Anselmo L. S. Melo <anselmo.melo@openbossa.org>
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>