dm: remove impossible BUG_ON in __send_empty_flush

The flush_bio in question was just initialized to be empty, so there
is no way bio_has_data() will return true.  So remove stale BUG_ON().

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>