ff: Remove frobenius_map from Field trait

It is only used internally in the bls12_381 crate, and field extensions
aren't exposed anywhere in the Zcash stack.

ff/ff_derive/src/lib.rs

@@ -1266,11 +1266,6 @@ fn prime_field_impl(
                 #invert_impl
             }
 
-            #[inline(always)]
-            fn frobenius_map(&mut self, _: usize) {
-                // This has no effect in a prime field.
-            }
-
             #[inline]
             fn square(&self) -> Self
             {

ff/src/lib.rs

@@ -72,10 +72,6 @@ pub trait Field:
     /// failing if the element is zero.
     fn invert(&self) -> CtOption<Self>;
 
-    /// Exponentiates this element by a power of the base prime modulus via
-    /// the Frobenius automorphism.
-    fn frobenius_map(&mut self, power: usize);
-
     /// Returns the square root of the field element, if it is
     /// quadratic residue.
     fn sqrt(&self) -> CtOption<Self>;