Remove unused and superfluous imports

[faern]

Thank you a lot for creating this library! I have not really used it yet except toying around with the examples. But I'm looking for something that allows me to convert from spectrometer measured SPDs into various color spaces, so this looks just right!

I found that just cargo build (on latest stable Rust) caused three warnings. So I figured I could contribute some low hanging fruit potential improvements as a start.

[anderslanglands]

Thanks!