Simplify float string conversion function further

We now have a really simple function signature:

    pub fn from_str_radix_float<T: Float>(src: &str, radix: uint) -> Option<T>

By removing some of the arguments, we remove the possibility of some invalid states.
Deprecate {f32, f64}::from_str_hex

This is now covered by `FromStrRadix::from_str_radix`