K-Nearest Neighbor Filtering does a decent job. You can play with the parameters, but here are some quick settings I tried.
For each pixel, find the 49 nearest neighbors. Of those 49, average the 25 pixels nearest in intensity to the selected pixel. Replace the value of the selected pixel with that mean.
This can be done fairly efficiently with colfilt(). The result isn't perfect, but it's not terrible, either. I attached the result for enf.jpg.