dm bufio: delete unused and inefficient dm_bufio_discard_buffers

There is no user for this interface.  If in future it is needed it can
be reimplemented to walk the rbtree of buffers instead of doing
block-by-block lookups.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>