field: remove secp256k1_fe_equal_var

`fe_equal_var` hits a fast path only when the inputs are unequal, which is
uncommon among its callers (public key parsing, ECDSA verify).

src/group_impl.h

@@ -367,7 +367,7 @@ static int secp256k1_gej_eq_x_var(const secp256k1_fe *x, const secp256k1_gej *a)
 #endif
 
     secp256k1_fe_sqr(&r, &a->z); secp256k1_fe_mul(&r, &r, x);
-    return secp256k1_fe_equal_var(&r, &a->x);
+    return secp256k1_fe_equal(&r, &a->x);
 }
 
 static void secp256k1_gej_neg(secp256k1_gej *r, const secp256k1_gej *a) {
@@ -400,7 +400,7 @@ static int secp256k1_ge_is_valid_var(const secp256k1_ge *a) {
     secp256k1_fe_sqr(&y2, &a->y);
     secp256k1_fe_sqr(&x3, &a->x); secp256k1_fe_mul(&x3, &x3, &a->x);
     secp256k1_fe_add_int(&x3, SECP256K1_B);
-    return secp256k1_fe_equal_var(&y2, &x3);
+    return secp256k1_fe_equal(&y2, &x3);
 }
 
 static SECP256K1_INLINE void secp256k1_gej_double(secp256k1_gej *r, const secp256k1_gej *a) {