group: Hard-code a w-NAF window size of 4

We were already ignoring the actual bit length of the scalar, and
selecting the window size based on the maximum bit length, which
effectively hard-coded a window size of 4.

src/groth16/tests/dummy_engine.rs

@@ -417,10 +417,6 @@ impl Curve for Fr {
 }
 
 impl WnafGroup for Fr {
-    fn recommended_wnaf_for_scalar(_: &Self::Scalar) -> usize {
-        3
-    }
-
     fn recommended_wnaf_for_num_scalars(_: usize) -> usize {
         3
     }