blockchain: always select random outs using triangular distribution

It was only used by the older blockchain_storage.
We also move the code to the calling blockchain level, to avoid
replicating the code in every DB implementation. This also makes
the get_random_out method obsolete, and we delete it.

src/blockchain_db/lmdb/db_lmdb.cpp

@@ -1620,19 +1620,6 @@ uint64_t BlockchainLMDB::get_tx_block_height(const crypto::hash& h) const
   return *(const uint64_t*)result.mv_data;
 }
 
-//FIXME: make sure the random method used here is appropriate
-uint64_t BlockchainLMDB::get_random_output(const uint64_t& amount) const
-{
-  LOG_PRINT_L3("BlockchainLMDB::" << __func__);
-  check_open();
-
-  uint64_t num_outputs = get_num_outputs(amount);
-  if (num_outputs == 0)
-    throw1(OUTPUT_DNE("Attempting to get a random output for an amount, but none exist"));
-
-  return crypto::rand<uint64_t>() % num_outputs;
-}
-
 uint64_t BlockchainLMDB::get_num_outputs(const uint64_t& amount) const
 {
   LOG_PRINT_L3("BlockchainLMDB::" << __func__);

src/blockchain_db/lmdb/db_lmdb.h

@@ -157,8 +157,6 @@ public:
 
   virtual uint64_t get_tx_block_height(const crypto::hash& h) const;
 
-  virtual uint64_t get_random_output(const uint64_t& amount) const;
-
   virtual uint64_t get_num_outputs(const uint64_t& amount) const;
 
   virtual output_data_t get_output_key(const uint64_t& amount, const uint64_t& index);