Fix dead code found by Coverity (CID 419).

This was left over from an early draft of the microdescriptor code; it
began to populate the signatures array of a networkstatus vote, even
though there's no actual need to do that for a vote.

src/or/dirserv.c

@@ -2609,11 +2609,6 @@ dirserv_generate_networkstatus_vote_obj(crypto_pk_env_t *private_key,
   voter->nickname = tor_strdup(options->Nickname);
   memcpy(voter->identity_digest, identity_digest, DIGEST_LEN);
   voter->sigs = smartlist_create();
-  {
-    document_signature_t *sig = tor_malloc_zero(sizeof(document_signature_t));
-    memcpy(sig->identity_digest, identity_digest, DIGEST_LEN);
-    memcpy(sig->signing_key_digest, signing_key_digest, DIGEST_LEN);
-  }
   voter->address = hostname;
   voter->addr = addr;
   voter->dir_port = options->DirPort;