Remove `impl Default for Rotation`

Clippy complained that it could be derived, but what we actually want is
for developers to be explicit about what rotations they are using, for
which `Rotation::cur()` is provided.

CHANGELOG.md

@@ -24,6 +24,7 @@ and this project adheres to Rust's notion of
 
 ### Removed
 - `halo2::arithmetic::BatchInvert` (use `ff::BatchInvert` instead).
+- `impl Default for halo2::poly::Rotation` (use `Rotation::cur()` instead).
 
 ## [0.1.0-beta.1] - 2021-09-24
 Initial beta release!

src/poly.rs

@@ -288,12 +288,6 @@ impl<'a, F: Field, B: Basis> Mul<F> for Polynomial<F, B> {
 #[derive(Copy, Clone, Debug, PartialEq)]
 pub struct Rotation(pub i32);
 
-impl Default for Rotation {
-    fn default() -> Rotation {
-        Rotation(0)
-    }
-}
-
 impl Rotation {
     /// The current location in the evaluation domain
     pub fn cur() -> Rotation {