chore: remove #[must_use] attribute (#63)

Rust nightly reports:

    warning: `#[must_use]` has no effect when applied to a provided trait method

Hence I'm remove that attribute.

Author: vmx

src/gt.rs

@@ -300,7 +300,6 @@ impl Group for Gt {
         self.0.ct_eq(&Self::identity().0)
     }
 
-    #[must_use]
     fn double(&self) -> Self {
         Gt(self.0.square())
     }