unused pair counting function

[duncandc]

I don't think marked_cpairs.conditional_pairwise_distances.pyx is used in the package. Any qualms with removing it? @aphearin

[aphearin]

If it's not being used, then it should be deleted, so good catch. So long as the disappearance is clearly marked (make sure the module name and the main function name appear in the commit message and also the PR), we can always search for the code in case we decide we want to put it back in.

[duncandc]

Ok, let's do that for now. I will take care of it. It is old code that doesn't really follow the structure of the pair functions we do use.

I am actually coding up a replacement for it in a different branch for a different project. So it won't be forgotten.