elliptic-curve: remove ShrAssign bound on Scalars

Removes the bound from the `CurveArithmetic::Scalar` associated type.

I was attempting to figure out why this issue didn't result in any bugs:

RustCrypto/elliptic-curves#1319

It appears to be completely unused.

The easiest way to fix that bug is to delete all of the relevant code.

Author: tarcieri

elliptic-curve/src/arithmetic.rs

@@ -81,7 +81,6 @@ pub trait CurveArithmetic: Curve {
         + for<'a> Mul<&'a Self::ProjectivePoint, Output = Self::ProjectivePoint>
         + PartialOrd
         + Reduce<Self::Uint, Bytes = FieldBytes<Self>>
-        + ShrAssign<usize>
         + TryInto<NonZeroScalar<Self>, Error = Error>
         + ff::Field
         + ff::PrimeField<Repr = FieldBytes<Self>>;