block: Remove bdrv_swap() bdrv_swap() is unused now. Remove it and all functions that have no other users than bdrv_swap(). In particular, this removes the .bdrv_rebind callbacks from block drivers. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Reviewed-by: Max Reitz <mreitz@redhat.com> Reviewed-by: Alberto Garcia <berto@igalia.com> Reviewed-by: Fam Zheng <famz@redhat.com> Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>