Using this function would always be wrong because a dirty bitmap must
have a specific owner that consumes the dirty bits and calls
bdrv_reset_dirty_bitmap().

Remove the unused function to avoid future misuse.
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>