block/raw_bsd: Drop raw_is_inserted()

With the new automatically-recursive implementation of
bdrv_is_inserted() checking by default whether all the children of a BDS
are inserted, we can drop raw's own implementation.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
