remove forever-deprecated and hidden `f64` methods The methods `f64::is_positive` and `f64::is_negative` were deprecated since 1.0 and marked as `#[doc(hidden)]` in favor of `f64::is_sign_positive` and `f64::is_sign_negative`. They also only exist on `f64`, not on `f32`. But for some unknown reason, they have been marked as stable. This PR proposes to remove both methods as they were never a part of the documented API, assuming that a crater run finds no significant breakage.