block: Drop bdrv_is_encrypted()

The original purpose of bdrv_is_encrypted() was to inquire whether a BDS
can be used without the user entering a password or not.  It has not
been used for that purpose for quite some time.

Actually, it is not even fit for that purpose, because to answer that
question, it would have recursively query all of the given node's
children.

So now we have to decide in which direction we want to fix
bdrv_is_encrypted(): Recursively query all children, or drop it and just
use bs->encrypted to get the current node's status?

Nowadays, its only purpose is to report through bdrv_query_image_info()
whether the given image is encrypted or not.  For this purpose, it is
probably more interesting to see whether a given node itself is
encrypted or not (otherwise, a management application cannot discern for
certain which nodes are really encrypted and which just have encrypted
children).

Suggested-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Andrey Shinkevich <andrey.shinkevich@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>