Remove dependency on Calculus.jl

[ViralBShah]

Calculus.jl is not really maintained. I am not fully sure how deeply it is used in ApproxFun.jl, but if possible, it would be good to remove.

Of course, the package is stable and there is no urgency. I'm just noting it here for someone who may want to use this as a way to get involved.

[jishnub]

I think it's only used to get a list of functions to extend. We may potentially hard-code the list here, but if the package isn't being updated and the list isn't changing, then it probably doesn't matter much anyway.