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/modules/schnorrsig/main_impl.h

@@ -261,7 +261,7 @@ int secp256k1_schnorrsig_verify(const secp256k1_context* ctx, const unsigned cha
 
     secp256k1_fe_normalize_var(&r.y);
     return !secp256k1_fe_is_odd(&r.y) &&
-           secp256k1_fe_equal_var(&rx, &r.x);
+           secp256k1_fe_equal(&rx, &r.x);
 }
 
 #endif