group: Remove CurveProjective::Base and CurveAffine::Base

These associated types were completly unused. The only place we need
information about the base field of an elliptic curve is inside Jubjub
when operating over its coordinates to implement EC math inside the
circuit, and we can handle that either concretely, or with a future
trait specifically for that use-case.

src/groth16/tests/dummy_engine.rs

@@ -391,7 +391,6 @@ impl PrimeGroup for Fr {}
 
 impl CurveProjective for Fr {
     type Affine = Fr;
-    type Base = Fr;
 
     fn batch_normalize(p: &[Self], q: &mut [Self::Affine]) {
         assert_eq!(p.len(), q.len());
@@ -433,7 +432,6 @@ impl CurveAffine for Fr {
     type Compressed = FakePoint;
     type Uncompressed = FakePoint;
     type Projective = Fr;
-    type Base = Fr;
     type Scalar = Fr;
 
     fn identity() -> Self {