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/blockchain_db.h

@@ -104,7 +104,6 @@
  *   height      get_tx_block_height(hash)
  *
  * Outputs:
- *   index       get_random_output(amount)
  *   uint64_t    get_num_outputs(amount)
  *   pub_key     get_output_key(amount, index)
  *   tx_out      get_output(tx_hash, index)
@@ -463,9 +462,6 @@ public:
   // returns height of block that contains transaction with hash <h>
   virtual uint64_t get_tx_block_height(const crypto::hash& h) const = 0;
 
-  // return global output index of a random output of amount <amount>
-  virtual uint64_t get_random_output(const uint64_t& amount) const = 0;
-
   // returns the total number of outputs of amount <amount>
   virtual uint64_t get_num_outputs(const uint64_t& amount) const = 0;