remove unused function

[NormannK]

replace_nan_with_none(data) is not needed anymore since we don't have any NaN values. Another 13% speed increase.

[drbacke]

13% is impossible, this function is called once.
Also the use of zero values is not allowed, because zero is a valid value. But some results cannot be determined for the first entry. So there must be a placeholder, like None oder Null.

[NormannK]

change es requested in #132
13% is absolutely possible since we call this function ~160 times per optimization generation!
Please check if other values need to be set since we don't have a test for them or a documentation

[drbacke]

This should work. Thanks for changing :-)