removed the type checking for the customer defined distance metric.

I asked question in stackoverflow about How to use a customer distance metric for KNeighboursRegressor,
link: http://stackoverflow.com/q/20655287/807695

Scikit's builtin metrics don't support mixture types of dataset,so I have to write a customer metric function. But even I'm using my defined metric function, I still cann't use the KNeighboursRegressor because these two lines codes it force the input dataset to be pure float type,  I think this is kind of bug, so I removed these two lines.