Remove some logically dead code

Found by coverity. This is an artifact left over from the original
decaf import which generated the source code for different curves. For
curve 448 this is dead.
Reviewed-by: NRich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6373)

crypto/ec/curve448/eddsa.c

@@ -253,7 +253,6 @@ c448_error_t c448_ed448_verify(
         curve448_point_decode_like_eddsa_and_mul_by_ratio(pk_point, pubkey);
     curve448_scalar_t challenge_scalar;
     curve448_scalar_t response_scalar;
-    unsigned int c;
 
     if (C448_SUCCESS != error)
         return error;
@@ -291,9 +290,6 @@ c448_error_t c448_ed448_verify(
                                 &signature[EDDSA_448_PUBLIC_BYTES],
                                 EDDSA_448_PRIVATE_BYTES);
 
-    for (c = 1; c < C448_EDDSA_DECODE_RATIO; c <<= 1)
-        curve448_scalar_add(response_scalar, response_scalar, response_scalar);
-
     /* pk_point = -c(x(P)) + (cx + k)G = kG */
     curve448_base_double_scalarmul_non_secret(pk_point,
                                               response_scalar,