Anotehr support calculation bug

Hi,

I have found another problem with support calculation. When you build a conditional FP-tree, you try to clean it up to make the operations faster. This procedure is implemented incorrectly and causes problems with certain tree branches. I don't have very much time to fix the procedure itself, so I suggest just not to use it. The performance will be slightly worse, but there won't be problems with the logic. 

Don't have a test for it, as I was able to track the problem only on my large datasets, not small. Can't really spend much time now on trying to create a small dataset that will demonstrate an issue.