tools/virtio/ringtest: Remove bogus definition of BUG_ON()

BUG_ON(x) should raise an error if x is true, but assert(x) raises an
error if x is false. Remove this bogus definition of BUG_ON(), which
isn't used anyway.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>