refactor(crypto): remove code that is duplicated in secp256k1-zkp submodule

crypto/zkp_bip340.c

@@ -136,10 +136,6 @@ int zkp_bip340_sign_digest(const uint8_t *private_key_bytes,
   }
 
   if (result == 0) {
-    uint8_t zero[32] = {0};
-    if (!auxiliary_data) {
-      auxiliary_data = zero;
-    }
     if (secp256k1_schnorrsig_sign32(context_writable, signature_bytes, digest,
                                     &keypair, auxiliary_data) != 1) {
       result = -1;