Remove RealExt

These functions are of little utility outside a small subset of use cases. If people need them for their own projects then they can use their own bindings for libm (which aren't hard to make).
Clean up std::num::cmath and remove stale comments
auto merge of #11504 : bjz/rust/std-num-cleanups, r=brson

The gamma and bessel functions are of little utility outside a small specialized subset of use cases, and so they don't really make sense for inclusion in the standard library. The only reason they were included in the first place was to mirror libm, which is not a very good justification. If people need them for their own projects then they can make their own bindings to libm (which isn't too hard).