core: do not forbid txes without destination

This was spuriously forbidden in the recent subaddress patch,
which isn't inherently incompatible with these.

src/cryptonote_core/cryptonote_tx_utils.cpp

@@ -160,12 +160,6 @@ namespace cryptonote
   //---------------------------------------------------------------
   bool construct_tx_and_get_tx_key(const account_keys& sender_account_keys, const std::unordered_map<crypto::public_key, subaddress_index>& subaddresses, std::vector<tx_source_entry>& sources, const std::vector<tx_destination_entry>& destinations, const cryptonote::account_public_address& change_addr, std::vector<uint8_t> extra, transaction& tx, uint64_t unlock_time, crypto::secret_key &tx_key, std::vector<crypto::secret_key> &additional_tx_keys, bool rct)
   {
-    if (destinations.empty())
-    {
-      LOG_ERROR("The destinations must be non-empty");
-      return false;
-    }
-
     std::vector<rct::key> amount_keys;
     tx.set_null();
     amount_keys.clear();