Revert "Add integral overload for isNaN for use in generic code"

Reverts D-Programming-Language/phobos#2311

I should have looked at this sooner.

This is a terrible idea (!). To quote Don Clugston "I think it's a complete fantasy to think you can write generic code that will work for both floats and ints. The algorithms are completely different. [...] Producing generic code that works for both floats and ints is a fool's errand."

Once you start going down this path,

1.where do you stop? What about isInfinity?

2.it'll never work. Don is right.