Fix denoising sampling

The q_posterior function as implemented is not correct and is not needed for sampling. Recall in Ho et al. equation (7) is for x_0 which is not known during sampling.

And in Alg. 2 line 4 we just need the slightly denoised sample.

Issue flagged to me by @NielsRogge so all credit to him if I am not mistaken... and if I am mistaken then it's my fault!