rbd: Revert -blockdev parameter password-secret

This reverts a part of commit 8a47e8e.  We're having second thoughts
on the QAPI schema (and thus the external interface), and haven't
reached consensus, yet.  Issues include:

* BlockdevOptionsRbd member @password-secret isn't actually a
  password, it's a key generated by Ceph.

* We're not sure where member @password-secret belongs (see the
  previous commit).

* How @password-secret interacts with settings from a configuration
  file specified with @conf is undocumented.

Let's avoid painting ourselves into a corner now, and revert the
feature for 2.9.

Note that users can still configure an authentication key with a
configuration file.  They probably do that anyway if they use Ceph
outside QEMU as well.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1490691368-32099-10-git-send-email-armbru@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>
