Remove unused Float::is_sign_negative trait method

    warning: method `is_sign_negative` is never used
       --> src/lexical/num.rs:251:8
        |
    175 | pub trait Float: Number {
        |           ----- method in this trait
    ...
    251 |     fn is_sign_negative(self) -> bool;
        |        ^^^^^^^^^^^^^^^^
        |
        = note: `#[warn(dead_code)]` on by default

src/lexical/num.rs

@@ -248,7 +248,6 @@ pub trait Float: Number {
     fn from_bits(u: Self::Unsigned) -> Self;
     fn to_bits(self) -> Self::Unsigned;
     fn is_sign_positive(self) -> bool;
-    fn is_sign_negative(self) -> bool;
 
     /// Returns true if the float is a denormal.
     #[inline]
@@ -368,11 +367,6 @@ impl Float for f32 {
     fn is_sign_positive(self) -> bool {
         f32::is_sign_positive(self)
     }
-
-    #[inline]
-    fn is_sign_negative(self) -> bool {
-        f32::is_sign_negative(self)
-    }
 }
 
 impl Float for f64 {
@@ -432,9 +426,4 @@ impl Float for f64 {
     fn is_sign_positive(self) -> bool {
         f64::is_sign_positive(self)
     }
-
-    #[inline]
-    fn is_sign_negative(self) -> bool {
-        f64::is_sign_negative(self)
-    }
 }