Remove custom rolling function in sky subtraction QA

This is a minor issue that just aims to remove some custom code that could be handled by an existing library.

I've attached a plot from the sky subtraction QA showing the chi values as a function of sky percentile. The green line is from the custom rolling function, which has a range of 5-95 and steps of 10. Using existing functions (pandas in this case) is shown in black, with a range of 0-100 in steps of 10. The plot shows that the results are basically the same and that we can remove the custom function.