blockchain_db: harden code against invalid input types

If an invalid input type were to get to this, the code could
remove key images that might be present already in the chain,
which could allow a double spend, even if this is impossible
with the current code.

Reported by KeyboardWarrior.

src/blockchain_db/blockchain_db.cpp

@@ -216,15 +216,8 @@ void BlockchainDB::add_transaction(const crypto::hash& blk_hash, const std::pair
     }
     else
     {
-      LOG_PRINT_L1("Unsupported input type, removing key images and aborting transaction addition");
-      for (const txin_v& tx_input : tx.vin)
-      {
-        if (tx_input.type() == typeid(txin_to_key))
-        {
-          remove_spent_key(boost::get<txin_to_key>(tx_input).k_image);
-        }
-      }
-      return;
+      LOG_PRINT_L1("Unsupported input type, aborting transaction addition");
+      throw std::runtime_error("Unexpected input type, aborting");
     }
   }