carrot_impl: add consensus rule for unique output pubkeys in tx

Required by Carrot to mitigate burning bugs, described in section 4.3 of the Carrot spec:
https://github.com/jeffro256/carrot/blob/master/carrot.md#43-transaction-model

Also remove 0-out check in `check_output_types()`, which I added in and technically constitutes a retroactive network split.

Co-authored-by: j-berman <justinberman@protonmail.com>

src/cryptonote_core/tx_pool.cpp

@@ -110,15 +110,6 @@ namespace cryptonote
       return amount * ACCEPT_THRESHOLD;
     }
 
-    uint64_t get_transaction_weight_limit(uint8_t version)
-    {
-      // from v2, limit a tx to 50% of the minimum block weight
-      if (version >= 2)
-        return get_min_block_weight(version) / 2 - CRYPTONOTE_COINBASE_BLOB_RESERVED_SIZE;
-      else
-        return get_min_block_weight(version) - CRYPTONOTE_COINBASE_BLOB_RESERVED_SIZE;
-    }
-
     // external lock must be held for the comparison+set to work properly
     void set_if_less(std::atomic<time_t>& next_check, const time_t candidate) noexcept
     {